How Did The Matter In Our Universe Arise From Nothing?

On all scales in the Universe, from our local neighborhood to the interstellar medium to individual... [+] NASA, ESA, AND THE HUBBLE HERITAGE TEAM (STSCI/AURA)

How Did The Matter In Our Universe Arise From Nothing?

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by Ethan Siegel, Senior Contributor

January 5, 2018

When you look out at the vastness of the Universe, at the planets, stars, galaxies, and all there is out there, one obvious question screams for an explanation: why is there something instead of nothing? The problem gets even worse when you consider the laws of physics governing our Universe, which appear to be completely symmetric between matter and antimatter. Yet as we look at what's out there, we find that all the stars and galaxies we see are made 100% of matter, with scarcely any antimatter at all. Clearly, we exist, as do the stars and galaxies we see, so something (or process) must have created more matter than antimatter, making the Universe we know possible. But how did it happen? It's one of the Universe's greatest mysteries, but one that we're closer than ever to solving.

The matter and energy content in the Universe at the present time (left) and at earlier times... [+] NASA, MODIFIED BY WIKIMEDIA COMMONS USER 老陳, MODIFIED FURTHER BY E. SIEGEL

Consider these two facts about the Universe, and how contradictory they are:

• Every interaction between particles that we’ve ever observed, at all energies, has never created or destroyed a single particle of matter without also creating or destroying an equal number of antimatter particles.

• When we look out at the Universe, at all the stars, galaxies, gas clouds, clusters, superclusters and largest-scale structures everywhere, everything appears to be made of matter and not antimatter.

It seems like an impossibility. On one hand, there is no known way, given the particles and their interactions in the Universe, to make more matter than antimatter. On the other hand, everything we see is definitely made of matter and not antimatter. Here's how we know.

The production of matter/antimatter pairs (left) from pure energy is a completely reversible... [+] DMITRI POGOSYAN / UNIVERSITY OF ALBERTA

Whenever and wherever antimatter and matter meet in the Universe, there’s a fantastic outburst of energy due to particle-antiparticle annihilation. We actually observe this annihilation in some locations, but only around hyper-energetic sources that produce matter and antimatter in equal amounts, like around massive black holes. When the antimatter runs into matter in the Universe, it produces gamma rays of very specific frequencies, which we can then detect. The interstellar and intergalactic medium is full of material, and the complete lack of these gamma rays is a strong signal that there aren't large amounts of antimatter particles flying around anywhere, since that matter/antimatter signature would show up.

Whether in clusters, galaxies, our own stellar neighborhood or our Solar System, we have tremendous... [+] GARY STEIGMAN, 2008, VIA HTTP://ARXIV.ORG/ABS/0808.1122

• In our own galaxy’s interstellar medium, the mean lifetime would be on the order of about 300 years, which is tiny compared to the age of our galaxy! This constraint tells us that, at least within the Milky Way, the amount of antimatter that’s allowed to be mixed in with the matter we observe is at most 1 part in 1,000,000,000,000,000!

• On larger scales — of galaxies and galaxy clusters, for example — the constraints are less stringent but still very strong. With observations spanning from just a few million light-years away to over three billion light-years distant, we’ve observed a dearth of the X-rays and gamma rays we’d expect from matter-antimatter annihilation. What we’ve seen is that even on large, cosmological scales, 99.999%+ of what exists in our Universe is definitely matter (like us) and not antimatter.

This is the reflection nebula IC 2631, as imaged by the MPG/ESO 2.2-m telescope. Whether within our... [+] ESO

So somehow, even though we aren't entirely sure how, [the universe] had to have created more matter than antimatter in it's past. Which is made even more confusing by the fact that the symmetry between matter and antimatter, in terms of particle physics, is even more explicit than you might think. For example:

• every time we create a quark, we also create an antiquark,
• every time a quark is destroyed, an antiquark is also destroyed,
• every time we create-or-destroy a lepton, we also create-or-destroy an antilepton from the same lepton family, and
• every time a quark-or-lepton experiences an interaction, collision or decay, the total net number of quarks and leptons at the end of the reaction (quarks minus antiquarks, leptons minus antileptons) is the same at the end as it was at the beginning.

The only way we’ve ever made more (or less) matter in the Universe has been to also make more (or less) antimatter in an equal amount.

The particles and antiparticles of the Standard Model obey all sorts of conservation laws, but there... [+] E. SIEGEL / BEYOND THE GALAXY

But we know that it must be possible; the only question is how it happened. In the late 1960s, physicist Andrei Sakharov identified three conditions necessary for baryogenesis, or the creation of more baryons (protons and neutrons) than anti-baryons. They are as follows:

• The Universe must be an out-of-equilibrium system.

• It must exhibit C- and CP-violation.

• There must be baryon-number-violating interactions.

The first one is easy, because an expanding, cooling Universe with unstable particles (and/or antiparticles) in it is, by definition, out of equilibrium.

The second one is easy, too, since "C" symmetry (replacing particles with antiparticles) and "CP" symmetry (replacing particles with mirror-reflected antiparticles) are both violated in the weak interactions.

A normal meson spins counterclockwise about its North Pole and then decays with an electron being... [+] E. SIEGEL / BEYOND THE GALAXY

That leaves the question of how to violate baryon number.

In the Standard Model of particle physics, despite the observed conservation of baryon number, there isn't an explicit conservation law for either that or lepton number (where a lepton is a particle like an electron or a neutrino). Instead, it's only the difference between baryons and leptons, B - L, that's conserved. So under the right circumstances, you can not only make extra protons, you can make the electrons you need to go with them.

What those circumstances are is still a mystery, however. In the early stages of the Universe, we fully expect equal amounts of matter and antimatter to exist, with very high speeds and energies.

At the high temperatures achieved in the very young Universe, not only can particles and photons be... [+] BROOKHAVEN NATIONAL LABORATORY

As the Universe expands and cools, unstable particles, once created in great abundance, will decay. If the right conditions are met, they can lead to an excess of matter over antimatter, even where there was none initially. There three leading possibilities for how this excess of matter over antimatter could have emerged:

• New physics at the electroweak scale could greatly enhances the amount of C- and CP-violation in the Universe, leading to an asymmetry between matter and antimatter. Sphaleron interactions, which violate B and L individually (but conserve B - L) can then generate the right amounts of baryons and leptons. This could occur either without supersymmetry or with supersymmetry, depending on the mechanism.

• New neutrino physics at high energies, of which we have a tremendous hint, could create a fundamental lepton asymmetry early on: leptogenesis. The sphalerons, which conserve B - L, would then use that lepton asymmetry to generate a baryon asymmetry.

• Or, GUT-scale baryogenesis, where new physics (and new particles) are found to exist at the grand unification scale, where the electroweak force unifies with the strong force.

These scenarios all have some elements in common, so let's walk through the last one, just as an example, to see what could have happened.

In addition to the other particles in the Universe, if the idea of a Grand Unified Theory applies to... [+] E. SIEGEL / BEYOND THE GALAXY

If grand unification is true, then there ought to be new, super-heavy particles, called X and Y, which have both baryon-like and lepton-like properties. There also ought to be their antimatter counterparts: anti-X and anti-Y, with the opposite B - L numbers and the opposite charges, but the same mass and lifetime. These particle-antiparticle pairs can be created in great abundance at high enough energies, and then will decay at later times.

So your Universe can be filled with them, and then they'll decay. If you have C- and CP-violation, however, then it's possible that there are slight differences between how the particles and antiparticles (X/Y vs. anti-X/anti-Y) decay.

If we allow X and Y particles to decay into the quarks and lepton combinations shown, their... [+] E. SIEGEL / BEYOND THE GALAXY

If your X-particle has two pathways: decaying into two up quarks or an anti-down quark and a positron, then the anti-X has to have two corresponding pathways: two anti-up quarks or a down quark and an electron. Notice that the X has B - L of two-thirds in both cases, while the anti-X has negative two-thirds.

It's similar for the Y/anti-Y particles. But there is one important difference that's allowed with C- and CP-violation: the X could be more likely to decay into two up quarks than the anti-X is to decay into two anti-up quarks, while the anti-X could be more likely to decay into a down quark and an electron than the X is to decay into an anti-down quark and a positron.

If you have enough X/anti-X and Y/anti-Y pairs, and they decay in this allowed fashion, you can easily make an excess of baryons over antibaryons (and leptons over anti-leptons) where there was none previously.

If the particles decayed away according to the mechanism described above, we would be left with an... [+] E. SIEGEL / BEYOND THE GALAXY

In other words, you can start with a completely symmetric Universe, one that obeys all the known laws of physics and that spontaneously creates matter-and-antimatter only in equal-and-opposite pairs, and wind up with an excess of matter over antimatter in the end. We have multiple possible pathways to success, but it's very likely that nature only needed one of them to give us our Universe.

The fact that we exist and are made of matter is indisputable; the question of why our Universe contains something (matter) instead of nothing (from an equal mix of matter and antimatter) is one that must have an answer.

In this century, advances in precision electroweak testing, collider technology, and experiments probing particle physics beyond the Standard Model may reveal exactly how it happened. And when it does, one of the greatest mysteries in all of existence will finally have a solution.

Two Short Courses in "Matter - Antimatter"

ANTIMATTER

https://www.lhc-closer.es/taking_a_closer_look_at_lhc/0.antimatter

In the late 1920's Paul Dirac applied to quantum mechanics the ideas of Einstein's special theory of relativity. It followed from Dirac's equations that there must be states of negative energy.

Dirac suggested that a deficiency of an electron in one of these states would be equivalent to a short-lived positively charged particle, or a positron with the same mass as the electron, but intrinsically opposite in terms of electrical charge. In ordinary matter, a positron would rapidly encounter an electron and annihilate, resulting in a very short lifetime for it, but in a perfect vacuum a positron can live forever.

Actually, for every matter particle there corresponds an anti-matter particle. Anti-matter particles can correspond to matter particles in every respect except that any kind of charge (or quantum characteristic) is opposite.

When a particle and an anti-particle meet, they annihilate into pure energy and may give rise to energetic neutral force-carrier particles, such as gluons, photons or Z-bosons. Conversely, energetic force-carrier particles can give rise to matter particle/anti-particle pairs (pair production).

An unsolved mystery of cosmology is why the universe is dominated by matter rather than anti-matter. That's just what the LHCb experiment see violation CP).aims to find out.

The experimental High Energy Physics Group at the University of Santiago de Compostela (SPAIN) focuses its research activity in quark physics, trying to probe the limits of the Standard Model. The main current project is Flavour Physics and CP-violation at the LHCb experiment at CERN

The first ever creation of atoms of antimatter at CERN has opened the door to the systematic exploration of the anti world. The recipe for anti-hydrogen is very simple - take one antiproton, bring up one anti-electron, and put the latter into orbit around the former - but it is very difficult to carry out as antiparticles do not naturally exist on earth. They can only be created in the laboratory. In even rarer cases, the positron's velocity was sufficiently close to the velocity of the antiproton for the two particles to join - creating an atom of anti-hydrogen

Three quarters of our universe is hydrogen and much of what we have learned about it has been found by studying ordinary hydrogen. If the behaviour of anti-hydrogen differed even in the tiniest detail from that of ordinary hydrogen, physicists would have to rethink or abandon many of the established ideas on the symmetry between matter and antimatter. It is believed that antimatter "works" under gravity in the same way as matter, but if nature has chosen otherwise, we must find out how and why.

The next step is to check whether anti-hydrogen does indeed "work" just as well as ordinary hydrogen. Comparisons can be made with tremendous accuracy, as high as one part in a million trillion, and even an asymmetry on this tiny scale would have enormous consequences for our understanding of the universe. To check for such an asymmetry would mean holding the anti-atoms still, for seconds, minutes, days or weeks. The techniques needed to store antimatter are under intense development at CERN.

* * * * * * * *

Ask Ethan: What's So 'Anti' About Antimatter?

by Ethan Siegel

Senior Contributor

High-energy collisions of particles can create matter-antimatter pairs or photons, while... [+] FERMILAB

For every particle of matter that's known to exist in the Universe, there's an antimatter counterpart. Antimatter has many of the same properties as normal matter, including the types of interaction it undergoes, its mass, the magnitude of its electric charge, and so on. But there are a few fundamental differences as well. Yet two things are certain about matter-antimatter interactions:

(1) if you collide a matter particle with an antimatter counterpart, they both immediately annihilate away to pure energy, and

(2) if you undergo any interaction in the Universe that creates a matter particle, you must also create its antimatter counterpart.

So what makes antimatter so "anti," anyway? That's what Robert Nagle wants to know, as he asks:

On a fundamental level, what is the difference between matter and its counterpart antimatter? Is there some sort of intrinsic property that causes a particle to be matter or antimatter? Is there some intrinsic property (like spin) that distinguishes quarks and antiquarks? What what puts the 'anti' in anti matter?

To understand the answer, we need to take a look at all the particles (and antiparticles) that exist.

The particles and antiparticles of the Standard Model obey all sorts of conservation laws, but there... [+] E. SIEGEL / BEYOND THE GALAXY

This is the Standard Model of elementary particles: the full suite of discovered particles in the known Universe. There are generally two classes of these particles:

(1) the bosons, which have integer spins (..., -2, -1, 0, +1, +2, ...) and are neither matter nor antimatter, and

(2) the fermions, which have half-integer spins (..., -3/2, -1/2, +1/2, +3/2, ...) and must either be "matter-type" or "antimatter-type" particles.

For any particle you can think about creating, there are going to be a slew of inherent properties to it, defined by what we call quantum numbers. For an individual particle in isolation, this includes a number of traits you're likely familiar with, as well as some that you may not be familiar with.

These possible configurations for an electron in a hydrogen atom are extraordinarily different from... [+] POORLENO / WIKIMEDIA COMMONS

The easy ones are things like mass and electric charge.

An electron, for example, has a rest mass of 9.11 × 10-31 kg, and an electric charge of -1.6 × 10-19 C. Electrons can also bind together with protons to produce a hydrogen atom, with a series of spectral lines and emission/absorption features based on the electromagnetic force between them.

Electrons have a spin of either +1/2 or -1/2, a lepton number of +1, and a lepton family number of +1 for the first (electron) of the three (electron, mu, tau) lepton families. (We're going to ignore numbers like weak isospin and weak hypercharge, for simplicity.)

Given these properties of an electron, we can ask ourselves what the antimatter counterpart of the electron would need to look like, based on the rules governing elementary particles.

In a simple hydrogen atom a single electron orbits a single proton. In an antihydrogen atom a single... [+] LAWRENCE BERKELEY LABS

The magnitudes of all the quantum numbers must remain the same. But for antiparticles, the signs of these quantum numbers must be reversed. For an anti-electron, that means it should have the following quantum numbers [remember, mass stays the same but electrical charge reverses]:

• a rest mass of 9.11 × 10-31 kg,
• an electric charge of +1.6 × 10-19 C,
• a spin of (respectively) either -1/2 or +1/2,
• a lepton number of -1,
• and a lepton family number of -1 for the first (electron) lepton family.

And when you bind it together with an antiproton, it should produce exactly the same series of spectral lines and emission/absorption features that the electron/proton system produced.

Electron transitions in the hydrogen atom, along with the wavelengths of the resultant photons... [+] WIKIMEDIA COMMONS USERS SZDORI AND ORANGEDOG

All of these facts have been verified experimentally. The particle matching this exact description of the anti-electron is the particle known as a positron [a positive antielectron]. The reason why this is necessary comes when you consider how you make matter and antimatter: you typically make them from nothing. Which is to say, if you collide two particles together at a high enough energy, you can often create an extra "particle-antiparticle" pair out of the excess energy (from Einstein's E = mc2), which conserves energy.

Whenever you collide a particle with its antiparticle, it can annihilate away into pure energy. This... [+] ANDREW DENISZCZYC, 2017

But you don't just need to conserve energy; there are a slew of quantum numbers you also have to conserve! And these include all of the following:

• electric charge,
• angular momentum (which combines "spin" and "orbital" angular momentum; for individual, unbound particles, that's only "spin"),
• lepton number,
• baryon number,
• lepton family number,
• and color charge.

Of these intrinsic properties, there are two that define you as either "matter" or "antimatter," and those are "baryon number" and "lepton number."

In the early Universe, the full suite of particles and their antimatter particles were... [+] E. SIEGEL / BEYOND THE GALAXY

If either of those numbers are positive, you're matter. That's why quarks (which each have baryon number of +1/3), electrons, muons, taus, and neutrinos (which each have lepton number of +1) are all matter, while antiquarks, positrons, anti-muons, anti-taus, and anti-neutrinos are all antimatter. These are all the fermions and antifermions, and every fermion is a matter particle while every antifermion is an antimatter particle.

The particles of the standard model, with masses (in MeV) in the upper right. The Fermions make up... [+] WIKIMEDIA COMMONS USER MISSMJ, PBS NOVA, FERMILAB, OFFICE OF SCIENCE, UNITED STATES DEPARTMENT OF ENERGY, PARTICLE DATA GROUP

But there are also the bosons. There are gluons which have for their antiparticles the gluons of the opposite color combinations; there is the W+ which is the antiparticle of the W- (with opposite electric charge), and there are the Z0, the Higgs boson, and the photon, which are their own antiparticles.

However, bosons are neither matter nor antimatter. Without a lepton number or baryon number, these particles may have electric charges, color charges, spins, etc., but no one can rightfully call themselves either "matter" or "antimatter" and their antiparticle counterpart the other one. In this case, bosons are simply bosons, and if they have no charges, then they're simply their own antiparticles.

On all scales in the Universe, from our local neighborhood to the interstellar medium to individual... [+] NASA, ESA, AND THE HUBBLE HERITAGE TEAM (STSCI/AURA)

So what puts the "anti" in antimatter? If you're an individual particle, then your antiparticle is the same mass as you with all the opposite conserved quantum numbers: it's the particle that's capable of annihilating with you back to pure energy if ever the two of you meet.

• But if you want to be matter, you need to have either positive baryon or positive lepton number;

• if you want to be antimatter, you must have either negative baryon or negative lepton number.

Beyond that, there's no known fundamental reason for our Universe to have favored matter over antimatter; we still don't know how that symmetry was broken. (Although we have ideas.) If things had turned out differently, we'd probably call whatever we were made of "matter" and its opposite "antimatter," but who gets which name is completely arbitrary. As in all things, the [our] Universe is biased towards the survivors.

Pillars of Cosmology: How the Universe Got Its Structure [During the Thesan Cosmic Dawn]

click to enlarge

The universe’s first structure originated when some of the material flung outward by the Big Bang overcame its trajectory and collapsed on itself, forming clumps. A team of Carnegie researchers showed that denser clumps of matter grew faster, and less-dense clumps grew more slowly. The group’s data revealed the distribution of density in the universe over the last 9 billion years. (On the illustration, violet represents low-density regions and red represents high-density regions.) Working backward in time, their findings reveal the density fluctuations (far right, in purple and blue) that created the universe’s earliest structure. This aligns with what we know about the ancient universe from the afterglow of the Big Bang, called the Cosmic Microwave Background (far right in yellow and green). The researchers achieved their results by surveying the distances and masses of nearly 100,000 galaxies, going back to a time when the universe was only 4.5 billion years old. About 35,000 of the galaxies studied by the Carnegie-Spitzer-IMACS Redshift Survey are represented here as small spheres. Credit: The illustration is courtesy of Daniel Kelson. CMB data is based on observations obtained with Planck, an ESA science mission with instruments and contributions directly funded by ESA Member States, NASA, and Canada.

Pillar of Cosmology: ‘Elegant’ Solution

Reveals How the Universe Got Its Structure

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by CARNEGIE INSTITUTION FOR SCIENCE

April 28, 2020

A direct, observation-based test

of one of the pillars of cosmology

The universe is full of billions of galaxies — but their distribution across space is far from uniform. Why do we see so much structure in the universe today and how did it all form and grow?

A 10-year survey of tens of thousands of galaxies made using the Magellan Baade Telescope at Carnegie’s Las Campanas Observatory in Chile provided a new approach to answering this fundamental mystery. The results, led by Carnegie’s Daniel Kelson, are published in Monthly Notices of the Royal Astronomical Society.

“How do you describe the indescribable?” asks Kelson. “By taking an entirely new approach to the problem.”

“Our tactic provides new — and intuitive — insights into how gravity drove the growth of structure from the universe’s earliest times,” said co-author Andrew Benson. “This is a direct, observation-based test of one of the pillars of cosmology.”

The Magellan telescopes at Carnegie’s Las Campanas Observatory in Chile, which were crucial to the ability to conduct this survey. Credit: Photograph by Yuri Beletsky, courtesy of the Carnegie Institution for Science

The Carnegie-Spitzer-IMACS Redshift Survey was designed to study the relationship between galaxy growth and the surrounding environment over the last 9 billion years, when modern galaxies’ appearances were defined.

The first galaxies were formed a few hundred million years after the Big Bang, which started the universe as a hot, murky soup of extremely energetic particles. As this material expanded outward from the initial explosion, it cooled, and the particles coalesced into neutral hydrogen gas. Some patches were denser than others and, eventually, their gravity overcame the universe’s outward trajectory and the material collapsed inward, forming the first clumps of structure in the cosmos.

The density differences that allowed for structures both large and small to form in some places and not in others have been a longstanding topic of fascination. But until now, astronomers’ abilities to model how structure grew in the universe over the last 13 billion years faced mathematical limitations.

“The gravitational interactions occurring between all the particles in the universe are too complex to explain with simple mathematics,” Benson said.

So, astronomers either used mathematical approximations — which compromised the accuracy of their models — or large computer simulations that numerically model all the interactions between galaxies, but not all the interactions occurring between all of the particles, which was considered too complicated.

"A key goal of our survey was to count up the mass present in stars found in an enormous selection of distant galaxies and then use this information to formulate a new approach to understanding how structure formed in the universe,” Kelson explained.

The research team — which also included Carnegie’s Louis Abramson, Shannon Patel, Stephen Shectman, Alan Dressler, Patrick McCarthy, and John S. Mulchaey, as well as Rik Williams, now of Uber Technologies — demonstrated for the first time that the growth of individual proto-structures can be calculated and then averaged over all of space.

Doing this revealed that denser clumps grew faster, and less-dense clumps grew more slowly.

They were then able to work backward and determine the original distributions and growth rates of the fluctuations in density, which would eventually become the large-scale structures that determined the distributions of galaxies we see today.

In essence, their work provided a simple, yet accurate, description of why and how density fluctuations grow the way they do in the real universe, as well as in the computational-based work that underpins our understanding of the universe’s infancy.

“And it’s just so simple, with a real elegance to it,” added Kelson.

The findings would not have been possible without the allocation of an extraordinary number of observing nights at Las Campanas.

“Many institutions wouldn’t have had the capacity to take on a project of this scope on their own,” said Observatories Director John Mulchaey. “But thanks to our Magellan Telescopes, we were able to execute this survey and create this novel approach to answering a classic question.”

“While there’s no doubt that this project required the resources of an institution like Carnegie, our work also could not have happened without the tremendous number of additional infrared images that we were able to obtain at Kit Peak and Cerro Tololo, which are both part of the NSF’s National Optical-Infrared Astronomy Research Laboratory,” Kelson added.

The Basics of Dark Matter & Dark Energy

When a quark collides with its antiquark, the interaction produces energy in the form of moving particles, antiparticles, and energy. Because scientists can detect these particles and energy, they are not the mysterious dark energy.

Does dark energy come from antimatter?

Astronomy: Roen Kelly

Published: Monday, December 30, 2013

RELATED TOPICS: DARK ENERGY

Q: Could the energy produced during matter-antimatter annihilation in the early universe be dark energy? If not, where is that produced energy today?

Michael Lynch

Dallas, Texas

A: Astronomers see galaxies flying away from each other faster than expected. Some sort of energy — dubbed “dark energy” because we cannot identify what it is — must be causing this repulsion. We know that dark energy composes an amazing 68 percent of the universe, so it produces an extremely big effect. Normal matter — like stars, gas, and planets — is only 5 percent of the cosmos.

Scientists believe that the laws of physics are constant everywhere and at all times. Decades of experiments have tested this principle and shown that it is valid. Therefore, we can use our current theory to predict what happened at the Big Bang, even though no one was around to observe the universe’s beginning.

We can study and measure matter-antimatter annihilations in high-energy accelerators. For instance, when quarks interact with antiquarks, we can measure the newly produced particles that have energy we can observe. Thus, the collisions aren’t creating dark energy (we can’t see dark energy; we can only detect its effect). Accelerator experiments also show no hint of dark matter — the mysterious mass that makes up 27 percent of the cosmos.

Physicists believe that the simplest explanation of a scientific question is often the best, so we conclude that throughout the universe, when matter collides with antimatter, the interaction produces the same energy we see on Earth. If the laws of physics are constant over time, we assert that when the cosmos was full of matter and antimatter, starting around one-trillionth of a second after the Big Bang, collisions between them produced the same energy we see and feel every day.

As the universe expanded and cooled, the collisions’ energy went right back to where it came from: matter, antimatter, and energy. For some reason, however, there was one slight asymmetry; the process created more matter than antimatter, which is why we see only matter today. The early universe’s antimatter and matter simply converted into our matter. Thank goodness, too — an astronaut would not want to meet up with antimatter debris.

Howard Matis

Lawrence Berkeley National

Laboratory, California

* * * * * *

Two galaxy clusters collided to create the “Bullet Cluster,” shown here. Normal matter is shown in pink and the rest of the matter is illustrated in blue, revealing that dark matter dominates this enormous cluster. | X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.

What's the difference between dark matter and dark energy?

by Eric Betz

Published: Tuesday, March 3, 2020

RELATED TOPICS: DARK MATTER | DARK ENERGY | COSMOLOGY

Our universe is dominated by mysterious and invisible forms of matter and energy that have yet to be fully (or even adequately) understood.

Most of our universe is hidden in plain sight. Though we can’t see or touch it, most astronomers say the majority of the cosmos consists of dark matter and dark energy. But what is this mysterious, invisible stuff that surrounds us? And what’s the difference between dark energy and dark matter? In short, dark matter slows down the expansion of the universe, while dark energy speeds it up.

Dark matter works like an attractive force — a kind of cosmic cement that holds our universe together. This is because dark matter does interact with gravity, but it doesn’t reflect, absorb, or emit light.

Meanwhile, dark energy is a repulsive force — a sort of anti-gravity — that drives the universe’s ever-accelerating expansion.

Dark energy is the far more dominant force of the two, accounting for roughly 68 percent of the universe’s total mass and energy. Dark matter makes up 27 percent. And the rest — a measly 5 percent — is all the regular matter we see and interact with every day.

Dark matter cannot be photographed, but researchers can detect it and map it by measuring gravitational lensing. Its distribution is shown here in the blue overlay of the inner region of Abell 1689, a cluster of galaxies 2.2 billion light-years away. | NASA/ESA/JPL-Caltech/Yale/CNRS

Dark matter

In the 1930s, Swiss-born astronomer Fritz Zwicky studied images of the roughly 1,000 galaxies that make up the Coma Cluster — and he spotted something funny about their behavior. The galaxies moved so fast that they should simply fly apart. He speculated that some kind of “dark matter” held them together.

Decades later, astronomers Vera Rubin and Kent Ford found a similar phenomenon when they studied the rotation rates of individual galaxies. The stars at a galaxy’s outer edge should circle slower than stars near the center. That’s the way planets in our solar system orbit. Instead, they noticed that the stars on a galaxy’s outskirts orbit just as fast — or faster — than the stars closer in. Rubin and Ford had found more evidence that some invisible form of matter is apparently holding the universe together.

“Even stars at the periphery are orbiting at high velocities,” Rubin once explained in an interview with Discover. “There has to be a lot of mass to make the stars orbit so rapidly, but we can’t see it. We call this invisible mass dark matter.”

How did we discover dark matter? What is dark matter made of? How is dark matter different than dark energy? Astronomy’s free downloadable eBook, The Science Behind Dark Matter, contains everything you need to know about the elusive and invisible substance.

Astronomers now have many other lines of evidence that suggest dark matter is real. In fact, the existence of dark matter is so widely accepted that it’s part of the so-called standard model of cosmology, which forms the foundation of how scientists understand the universe’s birth and evolution. Without it, we can’t explain how we got here.

But that lofty status puts pressure on cosmologists to find definitive proof that dark matter exists and that their model of the universe is correct. For decades, physicists all over the world have employed increasingly high-tech instruments to try and detect dark matter. So far, they’ve found no signs of it.

A wide view of the local universe, spanning hundreds of millions of light-years, reveals the clumped and weblike structure of the cosmos, with strands of galaxies and immense voids. The Milky Way is just one of many points that make up the Virgo Supercluster. Rather than just empty, passive spaces, voids may hold clues to understanding dark matter, dark energy and galactic evolution. - Andrew Z. Colvin

Dark Energy

Astronomers have known that our universe is expanding for about a century now. Telescopic observations have shown that most galaxies are moving away from each other, which implies the galaxies were closer together in the distant past. As a result, the evidence piled up for the Big Bang. However, astronomers assumed that the combined gravitational pull of all the cosmos’ stars and galaxies should be slowing down the universe’s expansion. Perhaps it would even someday collapse back in on itself in a Big Crunch.

That notion was thrown out in the late 1990s, however, when two teams of astronomers spotted something that didn’t make any sense. Researchers studying supernovas in the the most distant galaxies discovered that distant galaxies were moving away from us faster than nearby galaxies. The universe wasn’t just expanding — the expansion was speeding up.

“My own reaction is somewhere between amazement and horror,“ astronomer Brian Schmidt, who led one of the two teams, told The New York Times in 1998. “Amazement, because I just did not expect this result, and horror in knowing that it will likely be disbelieved by a majority of astronomers — who, like myself, are extremely skeptical of the unexpected.“

This graphic illustrates how the universe expands over time. | Astronomy: Roen Kelly

But rather than refute it, subsequent observations have only made the evidence for dark energy more robust. In fact, some prominent critics of dark matter still accept the existence of dark energy.

Now, that doesn’t mean researchers know what dark energy is. Far from it. But they can describe its role in the universe, thanks to Albert Einstein’s theory of general relativity. Einstein didn’t know about dark energy, but his equations suggested new space can come into existence. And he also included a fudge factor in relativity called the cosmological constant, which he added — and later regretted — to keep the universe from collapsing inward. This idea allows space itself to have energy. However, scientists have still never actually seen this force on Earth.

Some theoretical physicists think there’s an entire dark realm of particles and forces out there, just waiting to be discovered. Whatever dark energy and dark matter are made of, they seem to be playing tug-of-war with our universe — both holding it together and pulling it apart.

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In 1998, researchers discovered that something was causing the expansion of the universe to speed up. | NASA’s Goddard Space Flight Center Conceptual Image Lab.

The Beginning to the End of the Universe: The mystery of dark energy

by Bruce Dorminey

 Published: Monday, February 1, 2021

RELATED TOPICS: DARK ENERGY | COSMOLOGY

This story comes from our special January 2021 issue, "The Beginning and the End of the Universe.” Click here to purchase the full issue.

The universe isn’t just expanding, it’s accelerating.

For almost a century, astronomers have known that the universe is expanding. Space-time is stretching itself out over billions of light-years, carrying the galaxies within it apart, like raisins embedded within a rising loaf of bread.

This steady expansion, pitted against the cosmos’ urge to collapse under its own gravity, means there are two main scenarios for how the universe will eventually end. These scenarios are dubbed the Big Crunch — where gravity overcomes expansion and the Big Bang occurs in reverse — and the Big Freeze — where gravity loses out to the expansion and all matter is isolated by unfathomable distances. (See “The Big Crunch vs. the Big Freeze,” page 50.)

For a while, researchers believed the universe’s fate was leaning toward the final scenario. But, in the late 1990s, astronomers discovered something unexpected that changed our understanding of the future of the universe: The most distant galaxies weren’t just moving away from us. They were accelerating.

A cosmological puzzle

This phenomenon was independently discovered by two teams of astronomers who were measuring distant supernovae to calculate the precise rate at which the universe was expanding, expecting to find it slowing down. Three of these scientists — Saul Perlmutter, Adam Riess, and Brian Schmidt — shared the 2011 Nobel Prize in Physics for their discovery.

The award-winning observations came from a survey of distant type Ia supernovae. Astronomers believe these explosions are triggered when a white dwarf — the dense remnant of a Sun-like star — accretes matter that pushes it over a physical mass limit. That limit is the same for all white dwarfs, making all type Ia supernovae the same true brightness. This property made these supernovae ideal standard distance markers, or standard candles, in the mid-1990s.

The two teams were actually looking back into time for the onset of cosmic deceleration: They were looking for the point in time at which gravity gained the upper hand over the cosmos’ rapid acceleration after the Big Bang. This moment would mark a turnaround, as gravity finally started to slow the rate at which galaxies and clusters of galaxies are pulled away from one another by the expansion of the universe.

Since scientists know the true brightness of the standard candles, they could anticipate how bright these distant supernovae would be if expansion was slowing down. But instead, they found the observed type Ia supernovae were 25 percent fainter than expected, proving that the universe’s expansion isn’t slowing down, but instead is speeding up.

By the end of 1998, both teams had submitted papers detailing their findings to academic journals. Perlmutter’s team published its paper in The Astrophysical Journal and Riess and Schmidt’s team published in The Astronomical Journal.

The conclusion of both: A large percent of the universe is made up of something previously undiscovered and unexpected. And this so-called dark energy is overpowering gravity and pushing space-time apart from within.

A lot of missing pieces

The composition of the universe is surprisingly tricky to pin down. Besides dark energy, space is also filled with an invisible form of matter known as dark matter. Astronomers now know that normal, visible matter makes up just 5 percent of the universe, while enigmatic dark matter and dark energy constitute 26 percent and 69 percent, respectively. In other words, astronomers don’t really understand what about 95 percent of the universe is really made of.

And even decades after their discovery, scientists still know shockingly little about the “dark” forces that rule our universe. “Understanding and measuring dark matter and dark energy is hard,” says Riess. “Imagine bumping around in a dark room, occasionally touching an elephant, having never seen one, and [trying to understand] what it is, what it looks like.”

But the dark room is the size of the universe and instead of touching the elephant, astronomers can only see the effects it has on other objects. Astronomers can see that dark matter gravitationally interacts with visible matter, so they suspect it to be made up of one or more unknown particles. Dark energy could be a fifth fundamental force of the universe. (The known four are: the weak force, the strong force, gravity, and electromagnetism.) But its exact properties are still a mystery, especially since dark energy seems to have randomly turned itself on. Riess says the most recent measurements show that dark energy really kicked off this acceleration about 5 billion to 6 billion years ago, and it’s been the dominant force ever since.

The simplest explanation for dark energy is that it is the intrinsic energy of space itself. Albert Einstein initially introduced such a concept to allow for a flat universe when laying out his theory of relativity. Einstein’s so-called cosmological constant is a repulsive force that counteracts the attractive force of gravity to allow for a universe that neither collapses nor expands. But, in the end, Einstein dismissed his concept after Edwin Hubble observed the universe expanding. The Nobel-winning supernovae work in the 1990s resurrected [Einstein's] cosmological constant and related it to dark energy.

Though astronomers cannot see dark matter directly, they can infer its location from observations. The distribution of dark matter (magenta) in supercluster Abell 901/902 is revealed in this photo by combining a visible light image of the supercluster and a dark matter map of the area. | VISIBLE LIGHT: ESO, C. Wolf (Oxford University, U.K.), K. Meisenheimer (Max-Planck Institute for Astronomy, Heidelberg), and the COMBO-17 collaboration. DARK MATTER MAP: NASA, ESA, C. Heymans (University of British Columbia, Vancouver), M. Gray (University of Nottingham, U.K.), M. Barden (Innsbruck), and the STAGES collaboration.

What lies ahead

To ultimately resolve this dark energy puzzle, Riess says scientists will need more than just measurements. The world’s best theoretical physicists have tried to work out a grand unified theory of physics that fully explains all aspects of the universe. But so far, gravity and quantum physics don’t seem to mesh, despite the fact that theorists believe their unification is essential to any theory that will also explain dark energy.

One thing scientists have been able to figure out, however, is the profound impact dark energy will have on the universe in the distant future.

If the contribution of dark energy grows as the universe ages, the universe will expand progressively faster over time. Other galaxies beyond our Local Group — which will have merged into a single giant galaxy nicknamed Milkomeda — will eventually be whisked out to such great distances that any far-future occupants of our solar system wouldn’t be able to view them.

In fact, Alexei Filippenko, an astronomer at University of California, Berkeley, who has worked with both teams that discovered dark energy, says, “If all records are lost, future civilizations might not ever know about other galaxies.” For them, he says, “[The universe] will be a cold, dark, lonely place.”