Dark Energy: Real and Overwhelming

Throw a baseball up toward the sky, and gravity will slow its travel from the moment it leaves your hand. For decades, astronomers assumed that the post-Big Bang universe worked the same way. Even though galaxies are flying apart as space expands, their motion should be decelerating as the eons go by, due to the pull of their gravity on each other.

But a decade ago, cosmologists discovered something totally unexpected. The expansion of the universe has not been slowing down but speeding up in the past few billion years. It's as if the baseball you threw upward, instead of slowing down, suddenly sprouted a rocket engine and took off toward the clouds.

Something akin to anti-gravity — dubbed "dark energy" for lack of a better term — has apparently been inflating space. The evidence? Extremely far-off galaxies are traveling away from us at the wrong speeds (as measured by their redshifts) for their distances (as measured by the brightnesses of supernovae within them).

Galaxy cluster Abell 85
Most of the matter in galaxy clusters is in the form of very hot gas, which emits copious X-rays. An example is the cluster Abell 85, shown here as a composite of an X-ray image (purple, from NASA's Chandra X-ray Observatory) and a visible-light image (from the Sloan Digital Sky Survey).
X-ray: NASA / CXC / SAO / A.Vikhlinin & others; Optical: SDSS
In recent years cosmologists seeking confirmation of dark energy's existence have looked for its effects another way: by analyzing distant clusters of thousands of galaxies with masses totaling up to a million billion (1015) Suns. Such galaxy clusters are the largest structures in the universe that are held together by their own gravity. They offer tantalizing hints that something like anti-gravity has indeed been retarding their evolution.

A team led by Alexey Vihklinin (Smithsonian Astrophysical Observatory) has discovered that very ancient galaxy clusters are much more massive than those that formed more recently. "What we find is that the growth of these structures has slowed down during the past 5½ billion years," Vihklinin explains, "and this is the unmistakable signature of dark energy."

The researchers used NASA's Chandra X-ray Observatory to map the hot, X-ray-bright gas filling dozens of clusters — some relatively young, others much older — to determine their masses. Their results won't be formally published until next February 10th's issue of the Astrophysical Journal, but a press briefing on Tuesday offered a preview (in easier-to-digest form!) about the years-long effort.

"A cluster's growth is really a competition between gravity's pull and accelerating expansion" of space, explains Smithsonian coauthor William Forman. And the Chandra observations indicate that younger clusters grew to be less massive than they should have, compared to the ones that came together early on. Roughly five billion years ago, with galaxies getting farther apart and their gravitational pull on each other weakening, the repulsive force of dark energy started to win out over gravity.

The work of Vihklinin and his team builds on earlier cluster work based on Chandra observations. In fact, Tuesday's announcement seems to echo a similar pronouncement linking galaxy clusters and dark energy made in mid-2004.

Our expanding universe
The expansion of the universe has been a tug-of-war between gravity (dominated by unseen "dark matter") and the repulsive force known as "dark energy." New results confirm that dark energy is winning out, and that the universe is now expanding at an accelerating rate.
NASA / Smithsonian Astrophysical Obs.
There's more to the story. The new result has buoyed cosmologists' confidence that they know dark energy's equation of state — that is, how its behavior changes, or doesn't, as the universe expands. It increasingly seems that the amount of dark energy in a given volume of space (a cubic centimeter, let's say) remains the same no matter how much space expands and how many cubic centimeters exist. This implies that dark energy is somehow associated with empty space itself, rather than being some kind of particles or field in space — which would thin out as space expands, as atoms and galaxies have done.

In other words, dark energy seems to match a notion conceived — and then rejected — by Albert Einstein nearly a century ago. Einstein invoked a gravity-defying "cosmological constant" to explain how a seemingly static universe (this was before cosmic expansion was even discovered) could resist collapse due to its own gravity.

The new results help to confirm that what we think of as "normal" matter — everything from stars and galaxies down to subatomic particles — represents only 4% of all the matter and energy that exists. The rest consists of 24% "non-baryonic dark matter" (made of something yet unknown, though particle physicists have their ideas), and 72% dark energy. The same ratio is indicated by the supernova-based galaxy distances measured a decade ago — and, with greater precision, by analyses of the cosmic microwave background radiation. It's yet another feather in the cap for the new era of "precision cosmology."

Besides Einstein's cosmological constant, theorists have proposed that dark energy might be explained in different ways: by an imperfection in general relativity requiring a modified law of gravity; or by an invisible energy field called "quintessence" that pervades the universe (though the equation-of-state value argues against this); or by an effect of unseen extra dimensions of space on a microscopic scale as implied by string theory.

"The simplest explanation," says David Spergel (Princeton University), "is that there's energy associated with empty space," as Einstein proposed and particle physicists have long speculated. In other words, Spergel quips, "even nothing weighs something" — and the weight of nothingness has a negative value. The universe contains so much nothing, that nothing's slight negative weight has begun pushing apart everything that's not already gravitationally bound. Spergel calls the new result a "triumph of general relativity."

Veteran cosmologist Michael Turner (University of Chicago) agrees, to a point. "Chandra's standalone evidence opens the door to a new technique," he tells Sky & Telescope. "All the observations are still consistent with the simple explanation of a cosmological constant, but 10 years later we’re still scratching our heads." There's still wiggle room for other ideas, Turner cautions, such as quintessence. "I think there’s a deepening appreciation of this being a very profound problem."

Now, more confident that dark energy really exists, cosmologists can imagine a universe destined literally to fly apart faster than the speed of light*. We'll always have the Milky Way's stars to gaze upon, as well as those of the Andromeda Galaxy (which is actually heading toward us) and, probably, other galaxies of our Local Group. But things farther away are not gravitationally bound to us, so some tens of billions of years from now, the accelerating expansion of space will carry them beyond all possibility of view.

---------------------

* Yes, space can expand faster than light. Einstein's rule that no matter or energy can move faster than light, a rule that has been confirmed ever more firmly for a century, only refers to motion through space.

Bear with me. Imagine that galaxies are like ships sitting dead in the water, with the water being space. Imagine that the ocean itself is expanding, due to a huge upwelling current, so that the ships move apart from each other. Even though each ship is sitting dead in the water, in this way it can end up moving away from the other ships much faster than its maximum hull speed — which only limits its velocity through the water.

We didn't say cosmology was easy. . . .

26 thoughts on “Dark Energy: Real and Overwhelming

  1. Rod

    The interpretation that dark energy in the universe today is causing space to expand more rapidly and space could expand >> c as the universe continues to age, suggests cosmologists are moving back to an inflation epoch to solve problems in the big bang paradigm – again. Folks need to remember that inflation was developed around 1980 to solve the horizon problem – the light-travel time problem in the big bang model concerning the smoothness of the cosmic microwave background radiation (CMBR) where 3D space expands >> c, only shortly after the inflation epoch to slow down. From this report, it appears we need another round of inflation to solve cosmology problems.

  2. Mathias

    If empty space acts as an opposing force to gravity, and in fact isn’t empty, rather the opposite. This “empty” space has to consist of something we’re not seeing (hello Aether again), because “nothing” can’t push. Either that or perhaps there is a dimension within our universe that’s expanding as a balloon (and we’re on its’ surface) and pushing everything apart. I kind of wonder what’s inside that balloon though. Maybe the fact that the universe is expanding is making it easier for this balloon to expand.

  3. Howard Knytych

    Expansion is a fundamental characteristic of the universe – it’s part of what defines the universe, as opposed to the notion that expansion is something the universe is doing.

  4. Jacques Millet

    What if your baseball is not attracted by the Earth but pushed towards Earth by a force from empty space? Two masses close together makes a shadow to this force and so, are pushed towards each other. As the Universe expands this force gets greater and galaxies fly away faster. Gravity and dark matter would’nt be opposing forces but the same force!…

  5. Robert

    I agree with Prof. Turner. This is “a very profound problem”. A new understanding of space and gravitational energy is needed. A complete paradigm shift will be needed and as in the past this will be the most difficult part of solving the problem of true spacial dynamics. But when that day does come we will see anti gravity devices and warp drives on spacecraft. Clues to these problems are being posted on the web now and we may find solutions here and there but someone needs to put the right pieces together. I can only wish??? Robert

  6. Robert

    Scientists seem to have taken up a rather disturbing tendency to try to make observations fit their theories, instead of modifying their theories to fit in with their observations. From paleontology, to archeology, to cosmology, to climate change, scientists cling desperately to their theories and try to “prove” them by observing what they want to see, rather than what is. What if the entire Big Bang theory is totally wrong. Then, suddenly, you can use the new observations to discover what the true origin of the Universe is, rather than wasting time trying to come up with ways of explaining observations that don’t fit the existing paradigm. We already know that the Big Bang isn’t possible within the confines of Einstein’s Relativity equations, and coupled with all of the other Cosmological observations that don’t fit, we need to either trash the Big Bang, admit we were wrong, and move on. Of course “we just don’t know” doesn’t read well in a paper, does it?

  7. Douglas Simpson

    Perhaps accelerating expansion is not so much a phenomenom which is being driven from the inside as these latest X-ray observations could suggest, but an opposite attracting force from outside our universe which is pulling it ever faster outward SEEMINGLY overwhelming the “overall” gravitational influence within. To suggest that nothing is something is to acknowledge that space is indeed affected by the presence of matter. But to suggest that this undefined something is a force which overwhelms gravity at larger scales is strident at best.

    I still feel that INSIDE our universe gravity is king and it is undeniably linked to matter. If the accelerating expansion of space is truly representative of a lessening of the overall influence of gravity, then the enormous structures which we observe at varying redshifts should not exist in my opinion. If the “normal”, “observable” baryonic matter in existence is truly only 4% of the universe, then gravity should have been overwhelmed long ago thus preclucing the formation of these giant structures.

    We may have to accept that the force driving this acceleration is from without and not from within. No matter how uncomfortable we may be with it’s implications.

  8. Nathaniel Sailor

    Yeah from all the astronomy books I read, this is no surprise dark energy is expaning the universe. But I’ll admit the space expaning faster than the speed of light does sound a little weird. But hay, it’s the fact.

  9. Rod

    This report has generated some interesting posts. Is dark energy a 5th force of nature and a fact? We must remember the energy density of space during inflation epoch is completely different than today when 3D space expanded >> c. The proposal here for dark energy changing the rate of the universe expansion is another modification or change to the force of empty space. It is clear that the present is not the key to the past. We should remember that dark energy is just one component of darkness in big bang cosmology to make the universe. Shortly after the formation of the CMB radiation, the universe plunges into the Cosmic Dark Ages for about 100 million years (during this period there were no stars or galaxies). We have dark matter as the 2nd component and the 3rd component is dark energy. These 3 components of darkness (cosmic dark ages, dark matter, and dark energy) somehow work together to make the universe we see in modern cosmology. Calling interpretations like this a fact seems like a stretch.

  10. Joel-MarksJoel Marks

    Kelly Beatty writes, “Something akin to anti-gravity — dubbed “dark energy” for lack of a better term — has apparently been inflating space.” I think a better term would be “levity.” Why not? It’s the opposite of gravity, and it would lighten things up a little bit. I’ve been suggesting this for a long time, but so far no takers. However Natural History magazine did publish the suggestion several years ago, as you can see: http://findarticles.com/p/articles/mi_m1134/is_9_110/ai_80061800
    So how does one go about getting a force of nature named?

  11. Bill

    With all the galaxies and stars that have been created since the Big Bang pouring out prodigeous quantities of energy, space is full of energy detectable in all wave lengths and like a hot air balloon without an envelope, you can expect the universe to expand. This force or pressure is everywhere and some must also be leaking out of the universe through the boundary. My guess is at some point in time, the fuel (hydrogen) for stars will run low, star formation will decrease and there will be less energy produced. At this point, the expansion will decrease. When the fuel and energy is gone (most of the stars extinct), the universe should start to contract.
    I’m sure somebody must have thought of this effect at some point, but I have never heard it mentioned. I would like some comment on this subject. Why is it ignored?

  12. Jonathan Hayes

    I’m not an expert, BUT…. back in the days I was taking elementary physics, we had “fudge factors” – the amount you added to or multiplied your answer by to get the answer you should have gotten. ‘Pears like we’ve got here a 96% “fudge factor” of “dark matter” plus “dark energy”. That’s awful big – seems like something’s basically wrong. Mebbe back to the drawing board time? Just a thought.

    “The universe is not only stranger than we know, it is also stranger than we can know.”

  13. Patrick

    How do we know at what rate the universe is expanding if there is no stationary point to go by? In other words, if everything is constantly moving what do we use as a starting point to determine what velocity and direction the universe is heading?

  14. Jim

    So, the Big Bang is a description of our evolving universe, from 10 to minus whatever second after creation. So called “space” is the separation of expanding matter, with acceleration. Since we are talking about a one of a kind event, why are we looking for usual terminology like energy to explain this acceleration? Expanding space IS the explanation, in itself. When was the last time we “created space” in any experiments, created anything, ex nihilio? Wasn’t Einstein’s blunder just a mathematical term, the entire expression/equation said to represent/explain/describe the de Sitter universe, but the fudge term itself not pointing to some force in nature. Not everything in reality can necessarily be atomized into convenient explanations. Creation, unique, unlike mere “explosions”, expands with acceleration. Period. We lack the initial conditions so we’re stuck with some holes in our explanation of everything – anyway.

  15. Vincent Cook

    As I noted in my comment on Robert Naeye’s article “Is Dark Energy Bad for Astronomy” (http://www.skyandtelescope.com/skytel/beyondthepage/35888949.html), dark energy is but one of several unfalsifiable hypothetical entities (dark matter and inflatons being a couple of others) that contemporary big bang models routinely invoke in order to maintain consistency with current evidence–in the case of dark energy, to explain excess dimming of high redshift supernovae. Never mind that the vacuum energy predicted by the well established theories of quantum physics and measured in various experiments differs from astrophysical conceptions of “dark energy” by 123 orders of magnitude; contemporary cosmology doesn’t seem inclined to allow negative results from the laboratory to get in the way of a good story.

  16. Carl

    The observations being discussed are (hold onto your seats) are consistent with a collapsing universe. To create these observations, we need our part of the universe to begin rushing inward toward a common point, while the outer edges of our universe is still expanding, not yet having fully lost the battle to gravity.

    When you view these events as symptoms of collapase, you get the exact same problem of having the outer regions “appearing” to be gaining speed as they rush outwardly. However, it is WE who are speeding up as we accelerate inwardly. Meanwhile, the light from galaxies in the outer regions is giving us the illusion that the expansion of the universe is speeding up. In truth, WE are crashing inward!

    Are we truly “crunching?” Well, it would seem a more logical way to explain the phenomenon we are observing than the posit that the rate of the expansion is speeding up.

    But they present this notion of speeding up as a partner in crime with dark energy.

    Look, since 1966 I have maintained that space has mass. Thus, the arrival of dark energy/dark matter strikes me as an absurd expanation of what we “see.” Space is the soup through which matter/energy swim.

    To understand where I’m comming from, please visit “THE TIME STREAM” at the following web address:

    http://home.gate.net/%7Ereberly2/sf/timezero.htm

    Pay special attention to the initial two episodes entitled, “Zero Time’ and “The Big Bang” respectively. The entire paper is brief, moves along quickly, and is written a mite tonge-in-cheek with dabs of humor to keep “heavy” things light. However, it is also shockingly truthful about our universe.

    The final episode, called “Future Time,” sends the reader on a brief fantasy that may be closer to reality than the glibness implies.

    Carl Eberly

  17. Carl

    {Note to whomever runs this place — I wish to resubmit my comments, as I caught and edited the typos.}

    The observations being discussed are (hold onto your seats) are consistent with a collapsing universe. To create these observations, we need our part of the universe to begin rushing inward toward a common point, while the outer edges of our universe are still expanding, not yet having fully lost the battle to gravity.

    When you view these events as symptoms of collapse, you get the exact same problem of having the outer regions “appearing” to be gaining speed as they rush outwardly. However, it is WE who are speeding up as we accelerate inwardly. Meanwhile, the light from galaxies in the outer regions is giving us the illusion that the expansion of the universe is speeding up. In truth, WE are crashing inward!

    Are we truly “crunching?” Well, it would seem a more logical way to explain the phenomenon we are observing than the posit that the rate of the expansion is speeding up.

    But they present this notion of speeding up as a partner in crime with dark energy.

    Look, since 1966 I have maintained that space has mass. Thus, the arrival of dark energy/dark matter strikes me as an absurd explanation of what we “see.” Space is the soup through which matter/energy swim.

    To understand where I’m coming from, please visit “THE TIME STREAM” at the following web address:

    http://home.gate.net/%7Ereberly2/sf/timezero.htm

    Pay special attention to the initial two episodes entitled, “Zero Time’ and “The Big Bang” respectively. The entire paper is brief, moves along quickly, and is written a mite tongue-in-cheek with dabs of humor to keep “heavy” things light. However, it is also shockingly truthful about our universe.

    The final episode, called “Future Time,” sends the reader on a brief fantasy that may be closer to reality than the glibness implies.

    Carl Eberly

  18. marc teitelbaum

    Very interesting view which you state: if I understand, jacques means that gravity is the force acting over great distances that forms a given mass through a kind of ‘negative’ pressure? As dark energy progresses, a given mass (star, solid,etc)forms from the ‘negative’ or ‘tail’ pressure of dark energy that focuses 360 degress on the forming mass, squeezing atoms to a central point. Is this what you meant or am I actually developing this idea. (Or am I myself out there too?) Seriously, this sounds intriguing. I invite responses.

  19. Rocket Queen

    What is the universe expanding into? All of the nothingness is in it, so how can it be expanding? If it is like a balloon, will it get too big and pop?

  20. Stephen

    The Universe expands everywhere, not just at the “edges” (which it may not have). The Universe’s expansion, like the Hubble constant, is a speed per unit distance. Not a speed. And the measured expansion rate today is something like (70 km/sec) / megaparsec. This is not a heck of a lot per 3 million light years – more or less the distance to the Andromeda Galaxy. It’s only over cosmological distances that this speed adds up to anything.

    We had astronomical distances – like the distances between stars. They’re huge, like light years. But a billion light years is another thing entirely. That’s cosmic.

    Instead of saying the the Universe is expanding faster than light, it’s much better to say that the Universe is expanding, and objects in it that are far enough away from each other are moving away from each other faster than light.

    OK, so it’s more words, but it doesn’t mislead.

  21. Peter WilsonPeter Wilson

    The “dark energy” enigma is a model oversimplification problem. If you model the Earth-Moon system as two point-masses in orbit, you will predict a static system. Yet a realistic model will predict expansion at an accelerating rate, both of which are observed. Likewise for the universe. Cosmologists have oversimplified the situation by modeling the cosmos with just two variables: density and expansion rate. A minimum of three are required, however. This can be seen with Newtonian gravity by considering a 2-body expansion problem. To characterize such a system, four variables are required: the masses of the two bodies; the rate of expansion; and the distance between them. We can eliminate one variable by making both masses the same size, but we cannot reduce a gravitational expansion problem to just two variables. Yet cosmologist claim to be able to have done exactly that: they claim to have modeled infinity with just density and expansion rate. Sorry, but this is mathematically impossible. The universe is not hiding a fifth force. When a realistic model of an infinite gravitational system is built, both the expansion and acceleration of the cosmos fall right out of it.

  22. Gaurav Karunakar

    I have read C.E.M.Joad’s book “God and Evil”. A few religions state the existence of an Evil presence opposed to God and a perpetual battle going on between Good and Evil. Most of the world’s problems are attributed to this struggle. I am not a religious person, but here is a force, the nature of which has not been conclusively found, operating within our knowledge. I hope the Cosmologists will soon find out the real nature of Dark Matter and Dark Energy.

  23. Dean Westerfield

    I read statements like “normal matter is 4% of all matter, 24% is dark matter and 72% is dark energy.” 72% by what measure – by weight, by volume? What exactly is it 72% of?

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