Comments on: Dark Energy: No Longer a Surprise

By: A New Challenge to Einstein? | Cosmic Variance | Discover Magazine

A New Challenge to Einstein? | Cosmic Variance | Discover Magazine — Mon, 12 Oct 2009 14:59:48 +0000

[...] it in different ways. Especially when it comes to astrophysics, where we need dark matter and dark energy to explain what we see, it makes sense to put Einstein to the most stringent tests we can [...]

By: Some Stuff You Might Want to Read « IBY’s Island Universe

Some Stuff You Might Want to Read « IBY’s Island Universe — Mon, 22 Dec 2008 04:54:50 +0000

[...] Cosmic Variance has some stuff about dark energy. (oooh, mysterious) By the way, would you like to read the movie scripts of your favorite movies [...]

By: Eyes Squared

Eyes Squared — Thu, 18 Dec 2008 05:48:18 +0000

John L. Winters Said:
December 16th, 2008 at 4:16 pm
So, looks like we are IN a blackhole. That was our big bang.

I’ve been saying this for years!!! I spoke about it with JC and at Fermi earlier this year. Apply the disintegration of matter crossing the event horizon of this much larger system to the progenesis of our universe and we have a full model of the system.

Big TOE here we come!!!

By: Low Math, Meekly Interacting

Low Math, Meekly Interacting — Wed, 17 Dec 2008 22:29:45 +0000

Am I wrong, or would such an intrinsic curvature (where I guess a cosmological constant is an analogous term to the “de Sitter radius” of the universe) solve some problems? Namely, some theorists would like extra symmetries to exactly cancel out SM contributions to the vacuum energy so that it’s zero, i.e. there is no cosmological constant. Since that doesn’t appear to be working, folks are stumped as to why it appears to be so very close to zero, when it should be much MUCH bigger if there is a non-zero value at all, short of invoking the Anthropic Principle. Well, if space has an intrinsic curvature, maybe, one could argue, these extra symmetries really DO cancel everything, and there really IS no c.c.

Of course, even then, maybe we’re still left stumped as to why the value of the de Sitter radius is what it is, and hence there’s no refuge here from anthropism. Hence, maybe it’s not the most well-motivated idea out there.

Just my idle curiosity. I can’t claim much of a legitimate opinion one way or the other.

By: Lawrence Crowell

Lawrence Crowell — Wed, 17 Dec 2008 21:15:18 +0000

Loaw math: I am in a bit of a minority on this, but I think the Lambda which is a pure Ricci curvature term may not be so much determined by a vacuum energy density and pressure, but rather this curvature determines the vacuum. We might expect on cosmological scales that any frame we impose, such as our Hubble frame, does not hold globally.

Lawrence B. Crowell

By: Leonard Ornstein

Leonard Ornstein — Wed, 17 Dec 2008 20:52:54 +0000

Sean:

The only observational evidence of the existence of black holes is the influence of the gravitational fields, of the otherwise unobservable matter ‘consumed’ by black holes, on matter on ‘our side’ of event horizons.

It’s often argued that since we can’t access putative events that might occur beyond the cosmological horizon or the event horizons of black holes with any kind of ‘light signal’, any discussion of “multi-verses”, or of phenomena or ‘existence’ on the ‘other side’ of such a horizon, belongs to science fiction. But, as noted, gravitational effects regularly extend across such horizons. Therefore theories involving massive ‘objects’ on the ‘other side’ of the cosmological horizon may constitute meaningful physics, if they predict observable effects within our universe that depend on the mass of such ‘objects’.

For example, appropriately-distributed mass, beyond the cosmological horizon, might be responsible for that excess acceleration of the galaxies nearest to the cosmic horizon which has been attributed to “dark energy”.

Have such possibilities been discussed? Do the Vikhilinin results conflict with such a possibility?

By: Sven

Sven — Wed, 17 Dec 2008 17:38:05 +0000

Have they taken the inertia of the matter into consideration? Just like static planets would fall into the star which they are situated in close proximity to, inertia could also potentially explain why the whole system hasn’t collapsed in on itself.

By: Shameless Booster of Friends

Shameless Booster of Friends — Wed, 17 Dec 2008 17:36:25 +0000

For the technical types in the audience:
A related paper appeared on the arxiv on the same day as the one discussed in this post. Given that we know the expansion history of the universe, ie we know that it’s 73% dark energy, 23% dark matter, etc, we can ask if the growth of structure shows evidence for departures from General Relativity, such as DGP or f(R) theories. [We're not talking MOND here]. These theories predict different behaviors for the ‘growth index’, a way of parameterizing modified GR theories. By counting the number of clusters at a given mass as a function of time, the same technique as the above paper, but with an independent dataset [which presented the above results last year], the authors were able to put the tightest constraints yet on the growth index. Breathe easy…GR is consistent with the data.

http://arxiv.org/abs/0812.2259

Definitely worth a read if you have a pet theory of modified GR!

By: Eugene

Eugene — Wed, 17 Dec 2008 17:19:13 +0000

I was about to post a comment about Ryan’s results, when I saw that he is already on the beat.

By: Low Math, Meekly Interacting

Low Math, Meekly Interacting — Wed, 17 Dec 2008 16:20:55 +0000

What’s the status of the idea that space-time has some intrinsic curvature that acts like lambda? If I understand correctly, the hypothesis is that space is “automatically” de Sitter with no extra “stuff” gravitating or anti-gravitating required. DOA? Gaining interest? Does it leave us still asking why space is that way (i.e. would our universe’s intrinsic curvature still seem fine-tuned)?

By: Lawrence Crowell

Lawrence Crowell — Wed, 17 Dec 2008 15:05:20 +0000

There is a difference between dark energy and dark matter. Dark matter is a cold nonluminous gravitating source which clumps around galaxies and clusters of galaxies. Dark energy is a homogeneous, or so it is thought, effect (I call it that because it might not be “energy” so much as some generalization of gravitation) that fills the universe.

Lawrence B. Crowell

By: Robin

Robin — Wed, 17 Dec 2008 12:17:40 +0000

@JC….

10% of the universe is quite a fascinating place…..

don’t be a grouch

and no nobody claims that they know what dark matter is, but the first steps towards understanding it has been taken.

By: JC

JC — Wed, 17 Dec 2008 10:27:20 +0000

“We shouldn’t lose our sense of wonder that we’re able to understand as much of the universe as we do …”

Uh? last time i looked physicists had no idea what 90% of the universe is made of. Giving something a name (dark energy, dark matter) doesn’t mean you understand it (see Feynman’s conversation as a child with his father). Nor does fiddling with some equations to “fit the data” automatically mean that you solved the riddle.

By: Lawrence Crowell

Lawrence Crowell — Wed, 17 Dec 2008 03:21:05 +0000

We seem to have about 75% for DE. Interesting that to capture a cluster right at the tipping point of formation vs expansion. This might be compared to “weighing the universe,” where the scales compare the relative “gravity” of DE and the cluster.

One thing I did spot, and I have not looked at the original paper yet, is the right hand plot has Omega_{/\} = 0 and Omega_m = .25. Is Omega_{total} = .25 for DM and ordinary matter?

Lawrence B. Crowell

By: Sean

Sean — Wed, 17 Dec 2008 01:38:04 +0000

Aaron is exactly right. The background cosmology is different for the second graph, so the interpretation of the observations in terms of number densities is different.

The 70% estimate is a holdover from the old-timey days of dark energy, indeed.

By: Aaron F.

Aaron F. — Wed, 17 Dec 2008 00:50:32 +0000

Sean – Come to think of it, where did that Omega_Lambda = 0.70 estimate come from in the first place? It doesn’t seem to be supported by any of the blobs on that plot, except maybe the Type Ia supernova data.

But the Type Ia supernova data came first… so maybe I just answered my own question…

By: Aaron F.

Aaron F. — Wed, 17 Dec 2008 00:41:19 +0000

Lab Lemming — Just a guess (I haven’t read the original paper, and I am not a cosmologist), but as Sean says, changing the amount of dark energy in your model affects the relationship between redshift and distance. In particular, it affects the volume you calculate for the region A < z < B, where z is redshift.

In the plots, the vertical axis is “number density of clusters,” and it’s given for two different redshift regions: 0.025 < z < 0.25 and 0.55 < z < 0.90. You can find the number density of clusters in a given redshift region by counting the clusters whose redshifts are in that range and then dividing by the calculated volume of the region. When you change your model, the calculated volume of the region will change, and the vertical coordinates of your data points will change as well!

This explains why the data points in the two plots have different vertical coordinates… but why do they also have different horizontal coordinates? The horizontal axis is “cluster mass,” and it’s given in units of “M_500.” If I recall correctly, M_500 is the mass of the highest-density region of the cluster—specifically, the region whose density is over 500 times the critical density of the universe. Depending on how you measure this, I think it could conceivably be model-dependent, although I’m not sure how.

So, in summary: the amount of dark energy you assume there is can change a lot of things in cosmology, including the values of some of your measurements!

On the other hand, I could be completely wrong.

By: Lab Lemming

Lab Lemming — Tue, 16 Dec 2008 23:19:20 +0000

OK, I’m confused.
You say that the two plots show the same data, compared to different models, one with and one without dark energy. But on these plots, the model curves seem to stay constant, while the data appears to move. What gives?

By: Sean

Sean — Tue, 16 Dec 2008 23:13:57 +0000

John– the dark energy fraction is not converging to 75%; it’s growing monotonically, and will approach 100%. Our measurements of the current dark energy fraction seem to be converging on 75% or thereabouts.

By: John R Ramsden

John R Ramsden — Tue, 16 Dec 2008 22:18:50 +0000

Or was it 25%