Follow humbly wherever and to whatever abyss Nature leads, or you shall learn nothing. -T.H. Huxley
We've spent a little bit of time talking about dark energy, including what we think of it, how we first discovered it, and how we knew that there wasn't just something out there blocking the light. It seems to be the latest abyss that Nature is leading us, so we needed to look beyond the type Ia supernova data and see what else the Universe was telling us.
Image credit: Don Dixon.
So what do we do? First off, we can try to measure how much matter is in the Universe independent of anything else. How do we do this? We use the most accurate method available, of course. This means taking giant surveys of galaxies and clusters of galaxies, combined with a knowledge of gravity.
Image credit: the 2dF galaxy redshift survey team.
Then you take this actual clustering data and you compare it with simulations of Universes with different matter compositions. You take a Universe with 10% matter, then you take another one with 20%, 30%, 40%, etc., and see which one matches the Universe you actually have in front of you.
Image credit: Millenium Simulation.
From clustering data, we can tell that the Universe has somewhere between 25 and 30% of its energy in the form of normal matter. Independent of any supernova data, we learn that most of the energy in the Universe is not normal matter.
So what's the rest of it? We need the cosmic microwave background to tell us that.
Image credit: WMAP team.
These tiny little fluctuations tell us a tremendous amount about what's in our Universe. Moreover, they tell us whether space in the Universe is curved positively like a sphere, flat like a sheet of paper, or curved negatively like a saddle.
These three different curvature cases would lead to the hot and cold spots looking different from one another, and the differences are striking. BOOMERANG was able to tell these cases apart.
Only the middle case -- a flat Universe -- holds up to the data. In fact, the limits are that if the Universe is curved, the amount of curvature is less than 2% of the total energy density. So we have not only supernovae, but clusters of galaxies and the cosmic microwave background too, all pointing towards the same Universe. One where it's spatially flat, full of about 25-30% matter, and where the remaining 70-75% is some mysterious form of energy. Seriously, all these different data sets point towards the same conclusion:
The Universe is mostly full of dark energy, which would need to exist even without the supernova data! It's a very unusual thing for all of these different sources of data to come in all at once, like they have over the past decade, and all support the same conclusion.
But this is what we've got, and it's supported from every angle. So take Huxley's advice, and follow Nature into the abyss of dark energy, or -- the horror -- you shall learn nothing.
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Congratulations for this blog .
Sigh...I still have a hard time wrapping my brain around the topology of space.
I've taken topology, and I still don't understand what's meant by open/closed and finite/infinite in physics.
Thank you, Ethan. I had no clue there was so much evidence.
When you say 30% "normal" matter, I assume you mean matter. That is baryonic and dark matter together.
Any ideas on what they are? Dark matter isn't in the Standard Model but perhaps DM particles could be 'detected' in a similar way to neutrinos. Dark energy OTOH, seems to need long time/distance n order to measure it's effect.
(bold mine)
Such is the mystery of dark energy that in comparison even dark matter becomes normal. But in reality dark energy is the norm. Ordinary matter, that small scattering of electrons and quarks with their capacity to form complex structures, is the exception.
As a huge factor (I've heard 95%) which we admittedly do not understand, dark energy/matter would seem to be a wild card. What's the chance that some of the more basic assumptions used in this remote sensing exercise are actually wrong? Are there presently any good alternative explanations for the WMAP data being what it is?
Could you explain the axis in the last diagram please?
Ethan, can you tackle the topology issue with a post at some point? I've heard many attempt to explain this, Laurence Krauss making the most sense to me in his "A Universe from Nothing" lecture. But in general, the topology issue and how the WMAP data points to a flat Universe, is lost on me.
Could you explain the axis in the last diagram please?
The X and Y axes refer to the density of matter and dark energy, respectively. They are normalized so that a value of 1 gives rise to a flat universe (the diagonal line so marked is Ω_Λ + Ω_M = 1). Gray shaded areas in the corners are excluded a priori: a universe in the upper left cannot be extrapolated backwards in time to a Big Bang, and presumably the region in the lower right would imply a universe that could never attain the presently observed size, if not one for which a Big Bang could not explode.
The orange, green, and blue regions are error bars obtained from three different data sets on where we are in this plane. The gray ellipses in the middle are the error ellipses from combining the three data sets.
A quick word from the village idiot... I don't even pretend to grasp the full extent of astrophysics or to understand fully the math. But in terms of broad concepts I can just about hang on. But I guess that's how we learn.
So - just wanted to say thanks for the blog... I really find it useful that you break some of these topics into bite-size chunks and give those of us with an interest a glimpse into the wonder of it all
http://www.scientificamerican.com/article.cfm?id=splitting-time-from-sp…
rather suggests that dark matter may not exist after all, and it's all a wild goose chase. To me, it would make so much more sense - it also disposes of the Bag Bang.
Any comments on this, Ethan?
Morg, you're welcome!
Davem, the scientific american article contains an idea that is *barely* a hypothesis at this point. Is Horava's theory a reasonable theory of gravity? I.e., does it explain the things we already know? The answer is "no, not yet, but it possibly could be." So it's worth looking at, but it's way too early to bet on it. I imagine we will find that it has other catastrophic consequences, as all the other alternatives to dark matter have so far.
Never believe print headlines. Never ever ever believe the headlines from science magazines.
Ethan,
What is your take on the research behind this article in SciAm? The article is pretty superficial. My guess is that the prospects for the underlying research are not quite as strong as described.
http://www.scientificamerican.com/article.cfm?id=splitting-time-from-sp…
The article seems to indicate that research supports a cyclical universe (with the Big Bang construed as a Big Bounce) but wouldn't that idea have all sorts of problems with Entropy?
Ooops. I should have read the prior comments before posting. Mea Culpa.
just finished reading a "Pysics World" article on de Broglie's pilot wave theory (november issue).
near the end of the article they mention some things to look for to test the validity of the theory. one possibility is that early in the universes evolution, exotic particles may have condensed out that do not interact with "normal" matter that follows the present copenhagen interpretation of quantum mechanics. it also mentioned that anisotropies might have formed which could be detected in the CMB. my first thought was that perhaps the theory could help explain the lack of anti-matter, then i thought nah, it explains DARK MATTER!!!
(then i thought i could make millions writing self help books on how to harness your inner pilot wave, exist in a non-local non-equilibrium quantum state enabling you to communicate superluminally on facebook)
Nice summary and diagram of current simulations (models).
But the assumptions behind these simulations are important. For example, all of these models assume that universe = our "visible universe"; specifically that there are only 3 spatial dimensions, not for example 10 spatial dimensions of string theory. Also, I don't have any idea what assumptions these models make about black holes. Effectively a black hole still gravitates like ordinary matter; but is it dark matter from a simulation point of view or ordinary matter. The extra spatial dimensions of string theory are important because if matter inside a black hole somehow leaves our "visible universe" then it somehow becomes extra dimensional; and this suggests an extra-dimensional dynamic exchange (not considered in these simulations) of matter and antimatter that is not incorporated in (I suppose) any of these simulations. Are the possible dynamics of including black holes and white holes (described by Roger Penrose as "we may indeed envision more localiuzed "little bangs" called white holes, which are more precisely the time-reverse of black holes") included in any of these simulations. And in the referenced simulations, the energy of "dark energy" is stuff confined in 3-D rather than a dynamic exchange in 10-spatial dimensions. Of course, cosmologist refuse discussion about extra dimensions except in code phrases like baby universes, black holes, big bangs. And finally, such simulations use Newtonian gravitational theory; rather than general relativity which (as Fred I . Cooperstock explains in General Relativistic Dynamics: Extending Einstein's Legacy Throughout the Universe) accounts for much dark energy data without the "dark energy hypothesis".
Simulations are only as good as their assumptions. They are not theories; rather they are compressions of theory and data; kind of like putting the reality of the Grand Canyon on a postage stamp. A lot of important detail is missed.
If everything sees itself as the center of the universe, I donât understand how gravity can be a force that acts against expansion. Wouldnât gravity on a cosmic scale cancel out? If you use the expanding shell model of the universe (in which our 3-D space is represented as a 2-D shell and a 4th dimension is the radius) gravity would have to act inside the 4th dimension. If it did, wouldnât its inverse square law be invalid? Also, light doesnât pass through the 4th dimension (we canât see it), so why should gravity. Without gravity the universe can expand at an accelerating rate as is observed.
And in the referenced simulations, the energy of "dark energy" is stuff confined in 3-D rather than a dynamic exchange in 10-spatial dimensions. Of course, cosmologist refuse discussion about extra dimensions except in code phrases like baby universes, black holes, big bangs
Sometimes it's difficult to differentiate between what is scientific fact and that which may be new-age interpretations of reality based on scientific fact. Energy forces in the universe, specifically dark matter, may have scientific validity, but it is the conclusions we draw from the underlying science that may in fact lead us far from the pure science track. On subjects that are not open to interpretation, like copper repipe it is much simpler to come to agreed upon conclusions, rather than end up in existential arguments about true interpretation of the underlying science.
Effectively a black hole still gravitates like ordinary matter; but is it dark matter from a simulation point of view or ordinary matter. The extra spatial dimensions of string theory are important because if matter inside a black hole somehow leaves our "visible universe" then it somehow becomes extra dimensional; and this suggests an extra-dimensional dynamic exchange
I really like the blog and the way you write. Have a nice day