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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
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
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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
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.
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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.”
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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.
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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.“
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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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The Beginning to the End of the Universe: The mystery of dark energy
by Bruce Dorminey
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.
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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.”
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