Dark Energy BOSSes Around the Universe

The BOSS survey's detailed galaxy map is helping astronomers hone in on the nature, and existence, of dark energy.

Ever since the Big Bang, the expanding universe has been a battleground between two cosmic forces: gravity, which tries to hold the universe together, and dark energy, a mysterious repulsive energy that pervades all of space.

Illustration of baryon acoustic oscillations
An illustration of baryon acoustic oscillations (white rings), the sound waves that rippled across the early universe. The echo of these waves is imprinted on the cobweb-like structure of galaxies and helps astronomers measure how quickly the universe is expanding.
E.M. Huff, the SDSS-III team, and the South Pole Telescope team. Graphic by Zosia Rostomian.

For the first 8 billion years, gravity dominated, and cosmic expansion gradually slowed. But as the matter in the cosmos thinned out, gravity’s pull weakened, and about 6 billion years ago dark energy began winning the tug-of-war. The cosmic expansion stopped slowing and started to accelerate. That’s the situation today and, as best as we can tell, forevermore. But until we know for sure what dark energy actually is, the jury remains out.

Now, a project called the Baryon Oscillation Spectroscopic Survey (BOSS) is narrowing down the nature of dark energy with an amazing degree of precision. Astronomers presented the preliminary results from six papers at the National Astronomy Meeting in Manchester, England.

The preliminary results alone cover 130 pages, so here are the Cliff’s Notes:

Mapping the Visible Universe

The BOSS survey contains hundreds of thousands of galaxies. Viewed on scales of a hundred million light-years, galaxies are organized into gigantic, cobwebby structures — crowded walls and filaments separated by enormous, barren voids.

Astronomers can use this map to learn about the universe when it was much younger. Way before the first stars and galaxies had formed, light and matter coexisted in a hot, primordial soup. Huge sound waves rang through this soup, sloshing matter into higher-density and lower-density regions.

As the universe cooled and the earliest galaxies formed, an echo of these acoustic waves remained, imprinted into the galaxies’ stringy arrangements. This imprint acts as a standard ruler, a measurement with which astronomers can measure the expansion of the universe.

Measuring Dark Energy

The strangeness of dark energy might make astronomers squirm in their seats, but every measurement that’s been tried confirms its existence. The BOSS project is no exception — from the observations, astronomers conclude that dark energy makes up between 70.7 and 73.1 percent (at a 68% confidence level) of the universe’s matter-and-energy tally.

The BOSS papers also confirm that whatever dark energy may be, it appears to be unchanging over time: w, which quantifies dark energy’s pressure and density, is a constant of -1.03 (give or take 0.07). Neither of these results is surprising, but the degree of precision is unprecedented: the uncertainty is about five times smaller than previous studies.

“This is quite beautiful work that is delivering on the BOSS promise to provide precision constraints on dark energy and cosmological parameters,” says Niel Brandt (Penn State University), a BOSS scientist who was not involved in the most recent set of papers.

Testing Modified Gravity

Dark energy is called “dark” only because scientists don’t understand what it is. So, what if dark energy doesn’t exist at all — what if the problem lies with scientists’ understanding of gravity itself?

Well, BOSS tests that too.

If gravity itself becomes repulsive on very large scales, then that’s all scientists need to explain the accelerating expansion of the universe. Repulsive gravity would mean that Einstein’s general theory of relativity — a marvel of durability against every test that’s been thrown at it for nearly a century — breaks down on very large scales, an unsettling possibility.

Beth Reid (Lawrence Berkeley National Laboratory) tested for this breakdown by examining the growth of the bubblelike pattern of large-scale cosmic structures in the BOSS survey. General relativity predicts how fast galaxies should fall toward each other to create these structures. Reid measured the individual velocities of hundreds of thousands of galaxies, and found that they were falling exactly as described by gravity as we know it, even on enormous scales.

“We already knew that the predictions of general relativity are extremely accurate for distances within the solar system,” Reid says, “and now we can say that they are accurate for distances of 100 million light-years.”

It appears dark energy is here to stay.

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Monica Young

About Monica Young

Monica Young, a professional astronomer by training, is web editor of Sky & Telescope, where she creates, manages, and maintains website content, and contributes to the magazine.

31 thoughts on “Dark Energy BOSSes Around the Universe

  1. Peter WilsonPeter Wilson

    The Moon’s orbit is expanding at almost the same rate the universe is, and its expansion is accelerating. Both were predicted long before being measured. The expansion of the Moon’s orbit is due to an on-going process, not a big explosion long ago, and its acceleration is due to energy conservation in said process, not some mysterious dark energy. Cosmologist have over-simplified their model of the cosmos, hanging everything on just two parameters, density and expansion rate. Modeling gravitational systems, such as the Earth and Moon or the universe, requires more than two parameters. Both the expansion and acceleration of the universe could have predicted with a complete model. Like the Wizard of Oz, dark energy is quite a bit less than they imagine. The curtain will drop someday.

  2. Phil

    Say what? The lunar orbit is a long-understood example of tidal drag and conservation of momentum, and has absolutely nothing to do with Dark Energy, so why bring it up? While it’s true that Dark Energy may some day be viewed as an amusing dead end of late 20th century physics, it’s the best we have at this time. If you want to muddy the waters by carelessly mixing facts and opinion in order to confuse (or, illustrate your own confusion), I suggest you do your commenting on the Faux News site.

  3. Peter WilsonPeter Wilson

    Model oversimplification, that’s what. It is the easiest way in science to make a wrong prediction: oversimplify the model; leave some important factor(s) out. If you are calculating the Earth’s climate, and you omit the effects of humans burning fossil fuels, you will predict global cooling, not warming. Oversimplify the model, and you can make a wrong prediction. Again: Cosmologist have over-simplified their model of the cosmos, hanging everything on just two parameters, density and expansion rate. Modeling gravitational systems, such as the Earth and Moon or the universe, requires more than two parameters. They failed to predict the acceleration expansion of the cosmos, because they oversimplified their model.

  4. Randall

    I think you can "horn in" on a project, "hone" their experiment, and do your best to "home in" on the truth, but I don’t think you can "hone in" on anything. It’s an easy mistake to make.

  5. Bruce

    Peter Wilson, it’s clear that your opinions are strongly held. As I’ve posted earlier, if the moon’s orbital expansion is caused by the same thing that’s causing universal expansion then all orbital systems everywhere should be experiencing similar, that is, proportional, expansion. Gravitational systems would all be unwinding and would ultimately be unstable. This to me is unsettling to say the least. But, in all sincerity, I do wish for a better understanding of the mechanisms at work here, both on the earth/moon scale and universally. Monica Young’s above article outlines the conventional professional opinions on the universal side, but your comments caused me to seek a better understanding of the lunar situation. As Phil notes, tidal drag and conservation of angular momentum are at work here. Because the earth is rotating at a faster rate than the moon orbits, the moon experiences a slight acceleration, causing it’s orbital distance to slightly increase. Meanwhile, because the moon is orbiting slower than the earth spins, the earth experiences a tidal drag, causing it’s rotational rate to slowly decrease. The earth’s angular momentum is therefore being transferred to the moon, lifting it into a higher orbit. So, in the moon’s case the answer to the title of this post is that the rotational energy of the earth is doing the heavy lifting. My complements to the Wikipedia article “Tidal acceleration” for helping me understand this.

  6. Monica YoungMonica Young

    Randall, I have to "horn in" on your comment :). You made me curious, so I looked it up. Here’s what the American Heritage Dictionary (ahdictionary.com) has to say:

    hone in
    1. To move or advance toward a target or goal: The missiles honed in on the military installation.

  7. Peter WilsonPeter Wilson

    I did not say the CAUSE of the Hubble expansion is the same as in the Earth-Moon system (EMS). The point is, neither can be described with only 2 parameters, and expansion in both cases can be ascribed to on-going processes. Bruce has explained the cause of the EMS’s expansion, but not its acceleration, so here goes: Gravitational energy is proportional to 1/r, where r is distance. The figure @ http://www.flickr.com/photos/20293838@N00/173271226/ crudely illustrates the energy-vs-distance situation, with the solid-line being 1/r. The caption describes the situation for distant galaxies, but the shape of curve, 1/r, is the same. Because Earth is so much more massive than the Moon and spins considerably faster than the Moon orbits it, rotational energy is converted to gravitational energy at an approximately constant rate. To understand the acceleration, imagine the Moon moving up the curve at a constant vertical rate, that is, gaining energy at a constant rate. It will move horizontally faster and faster as the curve flattens, i.e. the expansion will accelerate. Understand why the EMS expands at an accelerating rate, and you are half-way to understanding why the universe expands at an accelerating rate.

  8. Bruce

    Peter, I apologize for drawing the wrong conclusions re your posts. I did think you were implying a causal link here. Thanks for helping us understand your interesting viewpoint. And I do see your point, too. Since the gravitational force between two objects is inversely proportional to the distance between them, if another force driving them apart remains constant there would be an accelerating rate of separation. In the moon’s case it’s orbital expansion rate would naturally increase as the attractive force lessens. It makes perfect sense that the same would be true for the universe at large. But, Peter, just what “on-going process” is supplying the energy for the Hubble expansion, the real heavy lifting?

  9. Tim

    In reference to the 1/r nature of the gravitational energy, the plot of Energy vs. distance does flatten out with increasing distance. So a specific increase in the Moon’s orbital energy (obtained by stealing Earth’s rotational energy), will produce a greater change in distance when the Moon is farther away than when it is closer. However, acceleration is the rate of change of velocity vs. *time*, not energy. In the EMS, the rate (in time) at which the Moon steals energy from the Earth will slow over time, as the Earth’s rotational period (our "day") slows to match the Moon’s orbital period. Eventually, Earth’s rotational period will match the Moon’s orbital period and the Moon can no longer rob energy. At that point it will cease moving away from Earth. (And Earth’s rotation will be synchronized to the Moon’s orbital period as the Moon’s rotation is today.) I don’t have the figures in front of me, but I don’t think the Moon’s movement from Earth is accelerating (getting faster as time goes on). I believe it is slowing over time. If it is not slowing now, it will begin to slow in the future. That does not appear to be the case with universal expansion.

  10. Bruce

    Tim, you raise some good points as well. How has the moon’s orbital distance changed over it’s history? 4 billion years ago when it was much closer to earth the tidal acceleration forces would have been much greater than today. Is the moon’s outward motion really accelerating as Peter Wilson has claimed? I’m not trying to start an argument here, I just seek a better understanding.

  11. Peter WilsonPeter Wilson

    Thank you, Bruce, we are halfway there. Tim raises an obvious objection. The key is, "rotational energy is converted to gravitational energy at an approximately constant rate." The conversion DOES slowly decrease with time as r increases, but not as fast as 1/r decreases, so the acceleration will continue for billions of years. As to what is providing the lifting energy in the cosmos, the hand-waving answer can be found by studying the same figure: http://www.flickr.com/photos/20293838@N00/173271226/ The remote galaxies are moving uphill, but recall that within our local region of space, in our local galaxy cluster, material is falling DOWN the curve to the left! This is illustrated, crudely, @ http://www.flickr.com/photo_zoom.gne?id=170753918&size=l When you appreciate that the 1/r energy curve gets steeper and steeper towards zero, you can grasp where the energy is coming from. The particles shown clumping in Figure 1 are on the steep part of the curve, losing a lot of energy very quickly, but the galaxies in Figure 2 are on the much flatter portion of the curve, gaining energy very slowly. If you demand more than hand-waving, I have done the math. The full-blown scientific paper, complete with an equation for dark energy and references, can be downloaded from the bio page of my website, http://www.peterwilson2012.org/biography.html Click on Dark Energy thesis. Monica: my thesis is only 6 pages, not 130. It would be well worth your time to read it and understand. It is not necessarily correct, but it is falsifiable: my equation is either right or wrong.

  12. Bruce

    Well, Peter, I’ve read your thesis paper, and, while I must admit that some of it went over my head, it was still well worth my time. I quote from your paper, “billions of years ago, when the moon was in a steeper part of Earth’s gravity well, the expansion was slower,” speaking of the lunar orbital expansion. Thus while the closer moon raised much higher tides in the young Earth, and therefore the force of the tidal dissipation energy was greater, the moon wouldn’t have been moving outward as fast back then because it would take more energy to “lift” the moon at earlier times. So, Peter, in your opinion, what force or forces is or are driving the accelerating universal expansion?

  13. Bruce

    Peter Wilson, after sleeping on it, your ideas are coming into better focus. First let me say that a personal idea that I’ve had for a long time is that the site for the universal expansion is in the larger intergalactic voids, and not space occupied by galaxies. So I awoke with a eureka moment; what your suggesting is that the observed gravitational contraction of galactic clusters is supplying the energy to increase the size of the larger voids, and at an accelerating rate. I’m over simplifying, but is this expression of your theory basically correct, Peter? And Monica, please, if you haven’t already done so, look at Peter’s work and tell us what you think. Could our friend here have just provided a key to solving one of the biggest mysteries in modern cosmology?

  14. Peter WilsonPeter Wilson

    Bruce: You have correctly expressed the thesis in a nutshell: "the observed gravitational contraction (within) galactic clusters is supplying the energy…" Hopefully, the recommendation from a former skeptic will motivate others to study it. By the way, the paper glosses over a lot. In the part that compares the dualistic model to the Friedmann solution, it helps to have a copy of said. Wikipedia provides an obfuscated version, where they have replaced H with a-dot (http://en.wikipedia.org/wiki/Friedmann_equations 2nd and 3rd equations from the top). Now that you understand the thesis, you are in a position to judge it. Unfortunately, as described in the Abstract, that means solving Einstein’s field equations for general relativity, explicitly including M, R[i], LD and eta (defined in the paper). And that means serious math: tensor-matrix algebra. TMA is over my head; I am not even sure if that is the correct terminology. According to the Big Bang theory (the TV show), there are only 25 people in the world who could do that kind of math, and they are all working on string theory at this time. I would like an answer to your last question as much as anyone. My equation is only a guess, but the thesis begs for an authoritative answer.

  15. Rod

    My observation – if dark energy is unchanging with time we see that the for the first 8 billion years after the big bang, dark energy had no effect but only the last 6 billion years after the big bang. When we consider inflation, 3D space was initially expanding >> c, apparently slowing down for the next 8 billion years after the big bang, and now in the past 6 billion years, 3D space is accelerating. This is indeed some strange energy that is dark. The big bang requires cosmic dark ages, dark energy, and dark matter to explain the origin of the universe without Genesis 1:1.

  16. Bruce

    Peter, your thesis does beg for an answer. And I was a skeptic. I’m not totally convinced yet, but I have revised my thinking quite a bit about your ideas. Why has my thinking changed? I realize upon further reflection that I was emotionally excited by your idea because I could see how it fit with a preconceived notion that I’ve had: the site for universal expansion is in the larger extra-galactic voids. Why do I think this? Fundamental core beliefs drive a persons thinking. As all who have read my posts by now know, I believe that the universe was created, that it didn’t come about by chance. As Einstein is quoted as saying, “God didn’t play dice with the universe.” I encourage all to look up Isaiah 45:18 find the stated reason behind creation. There the bible tells us that God intentionally formed the earth “to be inhabited.” (Don’t just take my word for it, look it up and see for yourself.) If the Earth was purposely formed to be inhabited, what about the rest of the universe? This question motivates my interest in astronomy. I’m not nearly as smart as he was, but you could say that my roll model as to a belief system was Isaac Newton. (continued on next comment)

  17. Bruce

    As we all know, Sir Isaac Newton was an extremely logical thinker. What is less remembered about him was that he was also highly religious. Newton applied his logical mind to the study of the Bible, as well as to the natural world. In this I feel he is very worthy of imitation. So, when presented with a scientific idea, I try to look at it logically as well as from a scriptural standpoint. Thus I can accept the big bang model, for it says that the universe had a beginning, just as Genesis 1:1 does. I like the finding of the accelerating expansion of the universe, because this fits with a number of verses that mention God’s “stretching out the heavens.” I don’t like the idea of the expansion continuing to the point of some “big rip,” effectively destroying habitability, for the Bible speaks of the Earth abiding forever. I fully realize that none of these personal preferences constitute scientific proof, but I suspect that Sir Isaac would understand my thinking, and I think he would agree with me. If only we had someone with his mathematical prowess to analyze the Wilsonian theory of universal expansion. Newton advanced every branch of mathematics of his day, so if he was here now he’d likely be working with tensor-matrix calculus, while also spreading the good news of God’s kingdom.

  18. Rod

    Bruce, you apparently reviewed Peter Wilson view of 3D space expanding between the Earth and Moon as the explanation. Q:Has Peter Wilson theory of the receding Moon been published in a peer reviewed journal? E.g. Science, Nature, ApJ or even creation science like ArJ or JOC. Q:Is the Wilsonian theory of 3D space expanding between Earth and Moon consistent with nearly 3,000 years of solar eclipse history concerning the tidal dissipation rate parameter? Bruce, remember the receding Moon was discovered through analysis of solar eclipse measurements and followed up in modern astronomy by investigating Assyrian records of solar eclipses. What we have now is nearly 3,000 years of measurement concerning the tidal dissipation rate parameter used to explain the receding Moon because of solar eclipse records and history. This does not use 3D space expanding.

  19. Bruce

    Rod, you might be drawing the same misunderstanding of Wilson’s expansion thinking that I and others have. He’s not saying that the Earth/Moon distance expansion is an effect of the larger Hubble expansion or it’s acceleration. What he’s suggesting I think is that as galactic clusters and larger systems are compressed the liberation of gravitational potential energy is a driving force producing an accelerated expansion of the larger intergalactic voids. He’s using the EMS expansion as an energy transfer analogy, but he’s not drawing a causal link. Reading his paper should clear this up. As to my calling it the “Wilsonian theory of universal expansion,” that was an attempt at humorous writing, comparing his idea to, say, Newtonian dynamics. Wilson’s already humbly acknowledge that he’s no Isaac Newton. Sadly, none of us are.

  20. Bruce

    Rod, you might be drawing the same misunderstanding of Wilson’s expansion thinking that I and others have. He’s not saying that the Earth/Moon distance expansion is an effect of the larger Hubble expansion or it’s acceleration. What he’s suggesting I think is that as galactic clusters and larger systems are compressed the liberation of gravitational potential energy is a driving force producing an accelerated expansion of the larger intergalactic voids. He’s using the EMS expansion as an energy transfer analogy, but he’s not drawing a causal link. Reading his paper should clear this up. As to my calling it the “Wilsonian theory of universal expansion,” that was an attempt at humorous writing, comparing his idea to, say, Newtonian dynamics. Wilson’s already humbly acknowledge that he’s no Isaac Newton. Sadly, none of us are.

  21. Peter WilsonPeter Wilson

    Rod: If it had been published, Sky&Tel would be the first to make news of it. The journals mentioned, and even arXiv.org, however, require PhD sponsorship before they will even look at a manuscript. I am presently seeking sponsorship, but consider the thesis: the mainstream model is missing two causal inputs, distance-between and contraction-energy. In layman’s terms: oops. Who wants to second that motion? Who wants to be the one to recommend publication of a thesis that thousands of his colleagues have forgotten two parameters in their model, and the oversight may explain dark energy? As you can imagine, I have not received a friendly response from any cosmologist that could act as a sponsor. I wish Sky&Tel would recognize this catch-22, and cut to the chase. The thesis begs for an answer. Sky&Tel ought to demand one.

  22. Bruce

    Peter, I feel your pain, dude. People are definitely drawing erroneous conclusions about your thinking, just as I did at first. Listen everyone! Wilson is not, Not, NOT saying that 3D space between the Earth and the Moon is expanding. He accepts the “long-understood” explanation, as Phil put it way back in the 2nd post in this thread, of the EMS expansion. What he’s suggesting is that on large enough scales, something similar to the EMS energy transfer could be happening, resulting in an accelerating expansion of the separation between large scale structures in the universe. We’ve all read the comparison of universal structure to that of a sponge. Picture the air pockets in a sponge being inflated somehow. What happens to the material between the air pockets? They’re compressed, not expanded, and if the rate of expansion of the pockets is greater than the rate of compression of the material between them the overall size of the sponge will expand. Doesn’t this remind you of what’s actually being observed universally? In our Local Group Andromeda and the Milky Way are observed to be cannibalizing smaller nearby galaxies while falling toward one another, so clearly this galactic cluster is contracting. Meanwhile if I remember right the local group as a whole is being drawn toward the Virgo cluster. If this was all there was of the universe we’d be talking about the big crunch, not universal, accelerating expansion. Even large clusters are observed to be colliding in places. Therefore in my pro-Newtonian opinion, what’s really being observed isn’t the wholesale expansion of all space, but only the expansion of the “enormous, barren voids” mentioned in Monica Young’s article. While Wilson’s explanation for why this part of space is expanding may be all wrong, the site of the expansion he’s talking about is exactly right.

  23. Peter WilsonPeter Wilson

    It is strange. Normally, the media loves a human-error story: the misshapen Hubble mirror; the mission to Mars that missed because someone forgot to convert English to metric; the loose-connector that caused the faster-than-light brouhaha. Apparently, opticians, computer programmers and technicians can err, but dark energy seems to be some kind of holy ground. My suggestion that astrophysicists have overlooked something seems to be sacrilege. Reportedly, there is tremendous scientific interest in understanding dark energy (see Monica’s report). I have offered duality as a possible explanation. It is not hand-waving; it is an equation in known parameters; it is falsifiable; it meets all the requirements of a scientific theory. Why can’t I get it evaluated? Why are Bruce and I the only ones interested in knowing if duality accounts for dark energy…or not?

  24. Bruce

    The stonewalling Wilson is facing from the gate keepers of scientific publication, if continued, will soon assume the aroma of scientific censorship. At least we live in a time when anyone can goggle “Causality and Duality in Cosmology” to find his thesis. People shouldn’t be fearful of it. Professional astronomers, have the intellectual courage to at least look at his idea. If he’s even partly right this would be a huge game changer in cosmology. Penzias scoured big teamed with another Wilson. By the way Peter, any relation to the Nobel Prize winner?

  25. Bruce Mayfield

    You’re my kind of scientist, Peter. Someday, I’d like to brag to my decedents that I once defended the deflator of the dark energy theory that threatened to rip the universe apart. It’s refreshing and likely very good for your mental health to see that you’ve managed to retain your sense of humor. Take heart, Wilson, for ultimately, the truth will out.

  26. Bruce

    Thanks but I think I’ll stay loyal to this one. I like their family friendly and no name calling rules. Keeps things nice and civilized. By the way, you might want to check your website’s inbox for feedback on this thread.

  27. Bruce Raffel

    I wonder if the elegant solution required for dark energy is not just gravity. I was watching the Science channel last night (January 5, 2014) and it seems to me if the current ideas are that there are possibly innumerable universes that the gravity from these alternate universes, and their effect on our universe may be why we are expanding rather than contracting. I see that some physicists think they can use date from sky maps to show "distortions" in the evolution of the sky, and those same distortions could simply be the effect of gravity from universes outside our own…hence, dark energy.

  28. Bruce Raffel

    I wonder if the elegant solution required for dark energy is not just gravity. I was watching the Science channel last night (January 5, 2014) and it seems to me if the current ideas are that there are possibly innumerable universes that the gravity from these alternate universes, and their effect on our universe may be why we are expanding rather than contracting. I see that some physicists think they can use date from sky maps to show "distortions" in the evolution of the sky, and those same distortions could simply be the effect of gravity from universes outside our own…hence, dark energy.

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