Quotes & Sayings

We, and creation itself, actualize the possibilities of the God who sustains the world, towards becoming in the world in a fuller, more deeper way. - R.E. Slater

There is urgency in coming to see the world as a web of interrelated processes of which we are integral parts, so that all of our choices and actions have [consequential effects upon] the world around us. - Process Metaphysician Alfred North Whitehead

Kurt Gödel's Incompleteness Theorem says (i) all closed systems are unprovable within themselves and, that (ii) all open systems are rightly understood as incomplete. - R.E. Slater

The most true thing about you is what God has said to you in Christ, "You are My Beloved." - Tripp Fuller

The God among us is the God who refuses to be God without us, so great is God's Love. - Tripp Fuller

According to some Christian outlooks we were made for another world. Perhaps, rather, we were made for this world to recreate, reclaim, redeem, and renew unto God's future aspiration by the power of His Spirit. - R.E. Slater

Our eschatological ethos is to love. To stand with those who are oppressed. To stand against those who are oppressing. It is that simple. Love is our only calling and Christian Hope. - R.E. Slater

Secularization theory has been massively falsified. We don't live in an age of secularity. We live in an age of explosive, pervasive religiosity... an age of religious pluralism. - Peter L. Berger

Exploring the edge of life and faith in a post-everything world. - Todd Littleton

I don't need another reason to believe, your love is all around for me to see. – Anon

Thou art our need; and in giving us more of thyself thou givest us all. - Khalil Gibran, Prayer XXIII

Be careful what you pretend to be. You become what you pretend to be. - Kurt Vonnegut

Religious beliefs, far from being primary, are often shaped and adjusted by our social goals. - Jim Forest

We become who we are by what we believe and can justify. - R.E. Slater

People, even more than things, need to be restored, renewed, revived, reclaimed, and redeemed; never throw out anyone. – Anon

Certainly, God's love has made fools of us all. - R.E. Slater

An apocalyptic Christian faith doesn't wait for Jesus to come, but for Jesus to become in our midst. - R.E. Slater

Christian belief in God begins with the cross and resurrection of Jesus, not with rational apologetics. - Eberhard Jüngel, Jürgen Moltmann

Our knowledge of God is through the 'I-Thou' encounter, not in finding God at the end of a syllogism or argument. There is a grave danger in any Christian treatment of God as an object. The God of Jesus Christ and Scripture is irreducibly subject and never made as an object, a force, a power, or a principle that can be manipulated. - Emil Brunner

“Ehyeh Asher Ehyeh” means "I will be that who I have yet to become." - God (Ex 3.14) or, conversely, “I AM who I AM Becoming.”

Our job is to love others without stopping to inquire whether or not they are worthy. - Thomas Merton

The church is God's world-changing social experiment of bringing unlikes and differents to the Eucharist/Communion table to share life with one another as a new kind of family. When this happens, we show to the world what love, justice, peace, reconciliation, and life together is designed by God to be. The church is God's show-and-tell for the world to see how God wants us to live as a blended, global, polypluralistic family united with one will, by one Lord, and baptized by one Spirit. – Anon

The cross that is planted at the heart of the history of the world cannot be uprooted. - Jacques Ellul

The Unity in whose loving presence the universe unfolds is inside each person as a call to welcome the stranger, protect animals and the earth, respect the dignity of each person, think new thoughts, and help bring about ecological civilizations. - John Cobb & Farhan A. Shah

If you board the wrong train it is of no use running along the corridors of the train in the other direction. - Dietrich Bonhoeffer

God's justice is restorative rather than punitive; His discipline is merciful rather than punishing; His power is made perfect in weakness; and His grace is sufficient for all. – Anon

Our little [biblical] systems have their day; they have their day and cease to be. They are but broken lights of Thee, and Thou, O God art more than they. - Alfred Lord Tennyson

We can’t control God; God is uncontrollable. God can’t control us; God’s love is uncontrolling! - Thomas Jay Oord

Life in perspective but always in process... as we are relational beings in process to one another, so life events are in process in relation to each event... as God is to Self, is to world, is to us... like Father, like sons and daughters, like events... life in process yet always in perspective. - R.E. Slater

To promote societal transition to sustainable ways of living and a global society founded on a shared ethical framework which includes respect and care for the community of life, ecological integrity, universal human rights, respect for diversity, economic justice, democracy, and a culture of peace. - The Earth Charter Mission Statement

Christian humanism is the belief that human freedom, individual conscience, and unencumbered rational inquiry are compatible with the practice of Christianity or even intrinsic in its doctrine. It represents a philosophical union of Christian faith and classical humanist principles. - Scott Postma

It is never wise to have a self-appointed religious institution determine a nation's moral code. The opportunities for moral compromise and failure are high; the moral codes and creeds assuredly racist, discriminatory, or subjectively and religiously defined; and the pronouncement of inhumanitarian political objectives quite predictable. - R.E. Slater

God's love must both center and define the Christian faith and all religious or human faiths seeking human and ecological balance in worlds of subtraction, harm, tragedy, and evil. - R.E. Slater

In Whitehead’s process ontology, we can think of the experiential ground of reality as an eternal pulse whereby what is objectively public in one moment becomes subjectively prehended in the next, and whereby the subject that emerges from its feelings then perishes into public expression as an object (or “superject”) aiming for novelty. There is a rhythm of Being between object and subject, not an ontological division. This rhythm powers the creative growth of the universe from one occasion of experience to the next. This is the Whiteheadian mantra: “The many become one and are increased by one.” - Matthew Segall

Without Love there is no Truth. And True Truth is always Loving. There is no dichotomy between these terms but only seamless integration. This is the premier centering focus of a Processual Theology of Love. - R.E. Slater


Note: Generally I do not respond to commentary. I may read the comments but wish to reserve my time to write (or write off the comments I read). Instead, I'd like to see our community help one another and in the helping encourage and exhort each of us towards Christian love in Christ Jesus our Lord and Savior. - re slater

Tuesday, December 23, 2014

Meet Christopher Skinner in Faith and Conversation

Christopher W. Skinner

Crux Sola: An Interview with Christopher W. Skinner

by Allan R. Bevere
December 22, 2014

Today's interview is with Christopher Skinner, who is Associate Professor of Religion at the University of Mount Olive in North Carolina. I got to know Christopher when we became contributors to a recent book published by InterVarsity Press, Jesus Is Lord, Caesar Is Not: Evaluating Empire in New Testament Studies. Chris blogs at Crux Sola (along with Nijay Gupta). I have found him to be a careful scholar and a person of deep and questioning Christian faith. If you have not read anything he has written, you need to do so.

Let the interview begin.


ARB: Tell us a little about yourself.

CWS: Well, that's a little open-ended! I guess I'll start with the most important things first. I have been married to my wife, Tara, since 1997 and our lives together revolve around caring for our three children, Christopher (14), Abby (12), and Drew (9). The four of them are the very best things in my life.

When I am not with my family, I serve as Associate Professor of Religion at the University of Mount Olive in North Carolina, where I have taught since 2010. I have a Ph.D. in Biblical Studies from the Catholic University of America in Washington, D.C. (2007), where I was privileged to spend significant time with a "who's who" of NT scholars, including Francis J. Moloney, Frank Matera, Joseph Fitzmyer, and Raymond Collins. 

I'm also a huge fan of the NFL franchise based in Washington, D.C., as well as the Baltimore Orioles…..which essentially means that I have been disappointed every football and baseball season for the better part of two decades.

ARB: What has been your faith journey, thus far? What are your core beliefs as a Christian?

CWS: I have talked about this a bit elsewhere. I was raised in a church culture that was quite conservative. That conservatism included how literally we interpreted the Bible and extended to social and political conservatism as well. While that foundation served me well in some respects throughout my youth and early adulthood, I ultimately found it to be far too restrictive in too many ways.

However, in that context I began to recognize something of a "calling" when I was about 14 years old, though I refused to acknowledge it until I was about 19. I guess you can say that I began to pursue that calling in my early 20s though it looks much different today than it did back then and certainly much different than I expected it to look when I first began the journey.

As far as my "core beliefs" go, I would say that I confess the Apostles' Creed (which we actually recite weekly at my church), even if there are some things in the creed that give me pause. I guess my core beliefs are somewhat Barthian in that I believe Jesus Christ must be the focus of a uniquely Christian faith.

ARB: How have you progressed as a scholar? What did you believe that you no longer can?

CWS: I would say that my progression as a scholar has been steadily incremental during this first full decade of my teaching career. Of course, the most dramatic growth took place during my doctoral program. Compared to most of my colleagues and friends, I had a fairly atypical experience. While many are able to devote themselves solely to a course of study during their doctoral program, I did not have that opportunity. I was married with children and serving on the full-time pastoral staff at a church in Baltimore while working on my Ph.D. at Catholic University. Though this schedule was demanding and created tremendous work (and stress) for me, it was an ideal situation in many ways.

The rigorous emphasis on developing competency in Greek, Hebrew, Coptic, Aramaic, Syriac, etc.-- which was a real hallmark of the program at CUA-- along with a focus on staying close to the text, helped me develop in more ways than I could have ever imagined. When you are that close to a text, its original language(s), and all the trappings of its interrelated cultures, previously "clear" theological formulations and doctrinal commitments can easily become very messy

This caused problems for my faith but also opened up other vistas that I never imagined. On the other hand, there was a real sense of urgency in my mind each week, to consider in fresh ways the significance of these texts for the community of faith I was serving. All of this made me much more sensitive to both ancient and modern contexts.

My progression as a scholar continues but it's not as dramatic. It helps to explain it like this: when you see your children every day, you don't get a true impression of how much they're actually growing. But when you look back at pictures from the previous year, you get a real sense of their growth and how dramatic it actually is. Every so often I'll read something I wrote a few years before or listen to something I previously taught or preached, and only then do I get a feel for how I've progressed. 

If I'm being completely honest with you, there are many things that I no longer believe, though I'm not sure it would be helpful to list all of them here. Perhaps it will be helpful for me to express it this way: There have been times when certain biblical texts spoke to me and edified me in clear and substantive ways-- ways that essentially determined major trajectories in my life. Today, those texts do not (and in most cases, cannot) speak to me with that same voice from the past. There's an inevitable sense of mourning and even disillusionment associated with that reality, though I am also driven to discover the new voices and different ways through which the text can speak. These days I find that I don't have a lot of answers, just a lot more questions

As I get older, my parents have gotten much smarter(!), and my mother has actually been immensely helpful as I wrestle with these issues. She's a very keen thinker with a great sensitivity toward the fragility of the human experience. I often bounce these things off of her to see if she has arrived at any more clarity than I have. Even if I don't arrive at answers, I find edification in the very conversation.

ARB: What have you written and published. What are you looking to write in the future?

CWS: My specific interests lie in the areas of Jesus and the gospel traditions. I have written or edited six books, all but one of which are devoted to issues in the interpretation of the gospels. I have just finished a manuscript for an introductory level textbook entitled, Reading John, which is set to be published in the Cascade Companions series early next year:

I am currently working on two projects, both of which are under contract. The first is a book I'm co-authoring with my blog mate, Nijay Gupta entitled, Across the Spectrum of New Testament Studies (Baker, 2017). That book aims to present an accessible and balanced introduction to the "spectrum" of viewpoints on key issues in New Testament studies. For each subject, between two and five positions are outlined, addressing the strengths and weaknesses of each stance.

The second project is an edited volume entitled, Johannine Ethics: The Moral World of the Gospel and Epistles of John (Fortress, 2017). Once these two projects are completed, I hope to write a few things for a wider audience than just students and scholars in the field of New Testament studies. I would like to address bigger questions for a much broader readership.

ARB: Too many persons, unfortunately think faith and scholarship are mutually exclusive. How do you bring the two together?

CWS: I don't think there's any formula for living in the midst of the tension that is created by the messy conclusions of biblical scholarship juxtaposed with the sometimes "tidy" world of faith commitments. I will admit that it is not easy for me. I often find sermons intellectually dishonest and condescending, which can make the tension even more unbearable. I have told my wife over and over that so often I feel like the father in Mark 9 who exclaims, "I believe. Help my unbelief."

I also think it is important to strive for intellectual honesty at every turn and admit when a belief either no longer makes sense or is no longer useful in light of what we are learning about the Bible and the world around us. In my thinking about theology I rely a lot upon the Wesleyan Quadrilateral. What I believe has to make sense in the contexts of reason, Scripture, tradition, and experience.

ARB: As a scholar and a churchman, what would you like to say to the church in general?

CWS: I would love for churches to stop baptizing their political ideologies in the name of Christianity and mining the pages of the Bible for proof texts that will support their political views and affiliations. Recently a study was released that found that an overwhelming majority of Christians in the United States were not opposed to torture. This leads me to wonder, does Christianity conceive of Jesus as the Prince of Peace or the Purveyor of Preemptive Justice? There's a lot more I could say here, but let’s fix one thing at a time.

ARB: If you could attend your ideal church, how would it look?

CWS: I have always found this to be such a difficult question. If we are to believe the accounts we read in the NT, Jesus was a very counter-cultural, non-institutional figure. So a major problem with ecclesiastical expressions of Christianity is that they build entire institutions with their own intentionally normative culture(s).

Ideally, I would love to be a part of an inclusive community that takes Jesus seriously. Too often in western churches the things we should take literally (like turning the other cheek, praying for our enemies) are spiritualized while things like "being forgiven so that we can go to heaven" (never explicitly mentioned in the NT) are literalized to the point of becoming the only basis of faith.

Also, I grew up in a Baptist church and there was always lots of food, so I guess an ideal church would also have an abundant supply of fresh, flaky biscuits. :-)

ARB: Thanks Chris for your time. If you would like to interact with Dr. Skinner, please visit his blog, Sola Crux.

Questions of Time: Is the Source to Time's Aarow based on Entropy or Gravitation?

2 Futures Can Explain Time's Mysterious Past

by Lee Billings
December 8, 2014

New theories suggest the big bang was not the beginning,
and that we may live in the past of a parallel universe

In the evolution of cosmic structure, is entropy or gravity the more dominant force? The answer to this question has deep implications for the universe's future, as well as its past.
Credit: NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University of California, Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team
Physicists have a problem with time.

Whether through Newton’s gravitation, Maxwell’s electrodynamics, Einstein’s special and general relativity or quantum mechanics, all the equations that best describe our universe work perfectly if time flows forward or backward.

Of course the world we experience is entirely different. The universe is expanding, not contracting. Stars emit light rather than absorb it, and radioactive atoms decay rather than reassemble. Omelets don’t transform back to unbroken eggs and cigarettes never coalesce from smoke and ashes. We remember the past, not the future, and we grow old and decrepit, not young and rejuvenated. For us, time has a clear and irreversible direction. It flies forward like a missile, equations be damned.

For more than a century, the standard explanation for “time’s arrow,” as the astrophysicist Arthur Eddington first called it in 1927, has been that it is an emergent property of thermodynamics, as first laid out in the work of the 19th-century Austrian physicist Ludwig Boltzmann. In this view what we perceive as the arrow of time is really just the inexorable rearrangement of highly ordered states into random, useless configurations, a product of the universal tendency for all things to settle toward equilibrium with one another.

Informally speaking, the crux of this idea is that “things fall apart,” but more formally, it is a consequence of the second law of thermodynamics, which Boltzmann helped devise. The law states that in any closed system (like the universe itself), entropy—disorder—can only increase. Increasing entropy is a cosmic certainty because there are always a great many more disordered states than orderly ones for any given system, similar to how there are many more ways to scatter papers across a desk than to stack them neatly in a single pile.

The thermodynamic arrow of time suggests our observable universe began in an exceptionally special state of high order and low entropy, like a pristine cosmic egg materializing at the beginning of time to be broken and scrambled for all eternity. From Boltzmann’s era onward, scientists allergic to the notion of such an immaculate conception have been grappling with this conundrum.

Boltzmann, believing the universe to be eternal in accordance with Newton’s laws, thought that eternity could explain a low-entropy origin for time’s arrow. Given enough time—endless time, in fact—anything that can happen will happen, including the emergence of a large region of very low entropy as a statistical fluctuation from an ageless, high-entropy universe in a state of near-equilibrium. Boltzmann mused that we might live in such an improbable region, with an arrow of time set by the region’s long, slow entropic slide back into equilibrium.

Today’s cosmologists have a tougher task, because the universe as we now know it isn’t ageless and unmoving: They have to explain the emergence of time’s arrow within a dynamic, relativistic universe that apparently began some 14 billion years ago in the fiery conflagration of the big bang. More often than not the explanation involves ‘fine-tuning’—the careful and arbitrary tweaking of a theory’s parameters to accord with observations.

Many of the modern explanations for a low-entropy arrow of time involve a theory called inflation—the idea that a strange burst of antigravity ballooned the primordial universe to an astronomically larger size, smoothing it out into what corresponds to a very low-entropy state from which subsequent cosmic structures could emerge. But explaining inflation itself seems to require even more fine-tuning. One of the problems is that once begun, inflation tends to continue unstoppably. This “eternal inflation” would spawn infinitudes of baby universes about which predictions and observations are, at best, elusive. Whether this is an undesirable bug or a wonderful feature of the theory is a matter of fierce debate; for the time being it seems that inflation’s extreme flexibility and explanatory power are both its greatest strength and its greatest weakness.

For all these reasons, some scientists seeking a low-entropy origin for time’s arrow find explanations relying on inflation slightly unsatisfying. “There are many researchers now trying to show in some natural way why it’s reasonable to expect the initial entropy of the universe to be very low,” says David Albert, a philosopher and physicist at Columbia University. “There are even some who think that the entropy being low at the beginning of the universe should just be added as a new law of physics.”

That latter idea is tantamount to despairing cosmologists simply throwing in the towel. Fortunately, there may be another way.

Tentative new work from Julian Barbour of the University of Oxford, Tim Koslowski of the University of New Brunswick and Flavio Mercati of the Perimeter Institute for Theoretical Physics suggests that perhaps the arrow of time doesn’t really require a fine-tuned, low-entropy initial state at all but is instead the inevitable product of the fundamental laws of physics. Barbour and his colleagues argue that it is gravity, rather than thermodynamics, that draws the bowstring to let time’s arrow fly. Their findings were published in October in Physical Review Letters.

The team’s conclusions come from studying an exceedingly simple proxy for our universe, a computer simulation of 1,000 pointlike particles interacting under the influence of Newtonian gravity. They investigated the dynamic behavior of the system using a measure of its "complexity," which corresponds to the ratio of the distance between the system’s closest pair of particles and the distance between the most widely separated particle pair. The system’s complexity is at its lowest when all the particles come together in a densely packed cloud, a state of minimum size and maximum uniformity roughly analogous to the big bang. The team’s analysis showed that essentially every configuration of particles, regardless of their number and scale, would evolve into this low-complexity state. Thus, the sheer force of gravity sets the stage for the system’s expansion and the origin of time’s arrow, all without any delicate fine-tuning to first establish a low-entropy initial condition.

From that low-complexity state, the system of particles then expands outward in both temporal directions, creating two distinct, symmetric and opposite arrows of time. Along each of the two temporal paths, gravity then pulls the particles into larger, more ordered and complex structures—the model’s equivalent of galaxy clusters, stars and planetary systems. From there, the standard thermodynamic passage of time can manifest and unfold on each of the two divergent paths. In other words, the model has one past but two futures. As hinted by the time-indifferent laws of physics, time’s arrow may in a sense move in two directions, although any observer can only see and experience one. “It is the nature of gravity to pull the universe out of its primordial chaos and create structure, order and complexity,” Mercati says. “All the solutions break into two epochs, which go on forever in the two time directions, divided by this central state which has very characteristic properties.”

Although the model is crude, and does not incorporate either quantum mechanics or general relativity, its potential implications are vast. If it holds true for our actual universe, then the big bang could no longer be considered a cosmic beginning but rather only a phase in an effectively timeless and eternal universe. More prosaically, a two-branched arrow of time would lead to curious incongruities for observers on opposite sides. “This two-futures situation would exhibit a single, chaotic past in both directions, meaning that there would be essentially two universes, one on either side of this central state,” Barbour says. “If they were complicated enough, both sides could sustain observers who would perceive time going in opposite directions. Any intelligent beings there would define their arrow of time as moving away from this central state. They would think we now live in their deepest past.”

What’s more, Barbour says, if gravitation does prove to be fundamental to the arrow of time, this could sooner or later generate testable predictions and potentially lead to a less “ad hoc” explanation than inflation for the history and structure of our observable universe.

This is not the first rigorous two-futures solution for time’s arrow. Most notably, California Institute of Technology cosmologist Sean Carroll and a graduate student, Jennifer Chen, produced their own branching model in 2004, one that sought to explain the low-entropy origin of time’s arrow in the context of cosmic inflation and the creation of baby universes. They attribute the arrow of time’s emergence in their model not so much to entropy being very low in the past but rather to entropy being so much higher in both futures, increased by the inflation-driven creation of baby universes.

A decade on, Carroll is just as bullish about the prospect that increasing entropy alone is the source for time’s arrow, rather than other influences such as gravity. “Everything that happens in the universe to distinguish the past from the future is ultimately because the entropy is lower in one direction and higher in the other,” Carroll says. “This paper by Barbour, Koslowski and Mercati is good because they roll up their sleeves and do the calculations for their specific model of particles interacting via gravity, but I don’t think it’s the model that is interesting—it’s the model’s behavior being analyzed carefully…. I think basically any time you have a finite collection of particles in a really big space you’ll get this kind of generic behavior they describe. The real question is, is our universe like that? That’s the hard part.”

Together with Alan Guth, the Massachusetts Institute of Technology cosmologist who pioneered the theory of inflation, Carroll is now working on a thermodynamic response of sorts to the new claims for a gravitational arrow of time: Another exceedingly simple particle-based model universe that also naturally gives rise to time’s arrow, but without the addition of gravity or any other forces. The thermodynamic secret to the model’s success, they say, is assuming that the universe has an unlimited capacity for entropy.

“If we assume there is no maximum possible entropy for the universe, then any state can be a state of low entropy,” Guth says. “That may sound dumb, but I think it really works, and I also think it’s the secret of the Barbour et al construction. If there’s no limit to how big the entropy can get, then you can start anywhere, and from that starting point you’d expect entropy to rise as the system moves to explore larger and larger regions of phase space. Eternal inflation is a natural context in which to invoke this idea, since it looks like the maximum possible entropy is unlimited in an eternally inflating universe.”

The controversy over time’s arrow has come far since the 19th-century ideas of Boltzmann and the 20th-century notions of Eddington, but in many ways, Barbour says, the debate at its core remains appropriately timeless. “This is opening up a completely new way to think about a fundamental problem, the nature of the arrow of time and the origin of the second law of thermodynamics,” Barbour says. “But really we’re just investigating a new aspect of Newton’s gravitation, which hadn’t been noticed before. Who knows what might flow from this with further work and elaboration?”

“Arthur Eddington coined the term ‘arrow of time,’ and famously said the shuffling of material and energy is the only thing which nature cannot undo,” Barbour adds. “And here we are, showing beyond any doubt really that this is in fact exactly what gravity does. It takes [entropic] systems that look extraordinarily disordered and makes them wonderfully ordered. And this is what has happened in our universe. We are realizing the ancient Greek dream of order out of chaos.”

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Additional References

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Why Does Dark Energy
Make the Universe Accelerate?

November 16, 2013

Peter Coles has issued a challenge: explain why dark energy makes the universe accelerate in terms that are understandable to non-scientists. This is a pet peeve of mine — any number of fellow cosmologists will recall me haranguing them about it over coffee at conferences — but I’m not sure I’ve ever blogged about it directly, so here goes. In three parts: the wrong way, the right way, and the math.

The Wrong Way

Ordinary matter acts to slow down the expansion of the universe. That makes intuitive sense, because the matter is exerting a gravitational force, acting to pull things together. So why does dark energy seem to push things apart?

The usual (wrong) way to explain this is to point out that dark energy has “negative pressure.” The kind of pressure we are most familiar with, in a balloon or an inflated tire, pushing out on the membrane enclosing it. But negative pressure — tension — is more like a stretched string or rubber band, pulling in rather than pushing out. And dark energy has negative pressure, so that makes the universe accelerate.

If the kindly cosmologist is both lazy and fortunate, that little bit of word salad will suffice. But it makes no sense at all, as Peter points out. Why do we go through all the conceptual effort of explaining that negative pressure corresponds to a pull, and then quickly mumble that this accounts for why galaxies are pushed apart?

So the slightly more careful cosmologist has to explain that the direct action of this negative pressure is completely impotent, because it’s equal in all directions and cancels out. (That’s a bit of a lie as well, of course; it’s really because you don’t interact directly with the dark energy, so you don’t feel pressure of any sort, but admitting that runs the risk of making it all seem even more confusing.) What matters, according to this line of fast talk, is the gravitational effect of the negative pressure. And in Einstein’s general relativity, unlike Newtonian gravity, both the pressure and the energy contribute to the force of gravity. The negative pressure associated with dark energy is so large that it overcomes the positive (attractive) impulse of the energy itself, so the net effect is a push rather than a pull.

This explanation isn’t wrong; it does track the actual equations. But it’s not the slightest bit of help in bringing people to any real understanding. It simply replaces one question (why does dark energy cause acceleration?) with two facts that need to be taken on faith (dark energy has negative pressure, and gravity is sourced by a sum of energy and pressure). The listener goes away with, at best, the impression that something profound has just happened rather than any actual understanding.

The Right Way

The right way is to not mention pressure at all, positive or negative. For cosmological dynamics, the relevant fact about dark energy isn’t its pressure, it’s that it’s persistent. It doesn’t dilute away as the universe expands. And this is even a fact that can be explained, by saying that dark energy isn’t a collection of particles growing less dense as space expands, but instead is (according to our simplest and best models) a feature of space itself. The amount of dark energy is constant throughout both space and time: about one hundred-millionth of an erg per cubic centimeter. It doesn’t dilute away, even as space expands.

Given that, all you need to accept is that Einstein’s formulation of gravity says “the curvature of spacetime is proportional to the amount of stuff within it.” (The technical version of “curvature of spacetime” is the Einstein tensor, and the technical version of “stuff” is the energy-momentum tensor.) In the case of an expanding universe, the manifestation of spacetime curvature is simply the fact that space is expanding. (There can also be spatial curvature, but that seems negligible in the real world, so why complicate things.)

So: the density of dark energy is constant, which means the curvature of spacetime is constant, which means that the universe expands at a fixed rate.

The tricky part is explaining why “expanding at a fixed rate” means “accelerating.” But this is a subtlety worth clarifying, as it helps distinguish between the expansion of the universe and the speed of a physical object like a moving car, and perhaps will help someone down the road not get confused about the universe “expanding faster than light.” (A confusion which many trained cosmologists who really should know better continue to fall into.)

The point is that the expansion rate of the universe is not a speed. It’s a timescale — the time it takes the universe to double in size (or expand by one percent, or whatever, depending on your conventions). It couldn’t possibly be a speed, because the apparent velocity of distant galaxies is not a constant number, it’s proportional to their distance. When we say “the expansion rate of the universe is a constant,” we mean it takes a fixed amount of time for the universe to double in size. So if we look at any one particular galaxy, in roughly ten billion years it will be twice as far away; in twenty billion years (twice that time) it will be four times as far away; in thirty billion years it will be eight times that far away, and so on. It’s accelerating away from us, exponentially. “Constant expansion rate” implies “accelerated motion away from us” for individual objects.

There’s absolutely no reason why a non-scientist shouldn’t be able to follow why dark energy makes the universe accelerate, given just a bit of willingness to think about it. Dark energy is persistent, which imparts a constant impulse to the expansion of the universe, which makes galaxies accelerate away. No negative pressures, no double-talk.

The Math

So why are people tempted to talk about negative pressure? As Peter says, there is an equation for the second derivative (roughly, the acceleration) of the universe, which looks like this:

(I use a for the scale factor rather than R, and sensibly set c=1.) Here, ρ is the energy density and p is the pressure. To get acceleration, you want the second derivative to be positive, and there’s a minus sign outside the right-hand side, so we want (ρ + 3p) to be negative. The data say the dark energy density is positive, so a negative pressure is just the trick.

But, while that’s a perfectly good equation — the “second Friedmann equation” — it’s not the one anyone actually uses to solve for the evolution of the universe. It’s much nicer to use the first Friedmann equation, which involves the first derivative of the scale factor rather than its second derivative (spatial curvature set to zero for convenience):

Here H is the Hubble parameter, which is what we mean when we say “the expansion rate.” You notice a couple of nice things about this equation. First, the pressure doesn’t appear. The expansion rate is simply driven by the energy density ρ. It’s completely consistent with the first equation, as they are related to each other by an equation that encodes energy-momentum conservation, and the pressure does make an appearance there. Second, a constant energy density straightforwardly implies a constant expansion rate H. So no problem at all: a persistent source of energy causes the universe to accelerate.

Banning “negative pressure” from popular expositions of cosmology would be a great step forward. It’s a legitimate scientific concept, but is more often employed to give the illusion of understanding rather than any actual insight.