# B Unsolved problems in cosmology

1. Mar 23, 2015

### wolram

To date AFAIK Dark matter and Dark energy are mathematical anomalies, What is the evidence for their existence and what is the latest thoughts as to what these hypothetical enteritis are?

2. Mar 23, 2015

### Doug Huffman

The evidences are the anomalous rotation of galaxies (DM) and the accelerating cosmological scale factor (DE).

3. Mar 23, 2015

### wolram

The evidences are the anomalous rotation of galaxies (DM) and the accelerating cosmological scale factor (DE).

This is not evidence for Dark Energy it is just a way of fiddling the books, its the same for Dark energy, there is no material evidence for either.

4. Mar 23, 2015

### wolram

I meant to write Dark Energy and Dark Matter
It seems the latest proposal to balance the books is Warm Dark Matter

http://chalonge.obspm.fr/colloque2015.html [Broken]

Last edited by a moderator: May 7, 2017
5. Mar 23, 2015

### phinds

I don't really understand asking for the "existence" of these "dark" things. The evidence that SOMETHING is happening in both cases is unequivocal and what we mean by the "dark" terms is simply "we don't KNOW what it is that is causing the things that are clearly happening".

SO ... asking if these things exist is tantamount to asking if a couple of weird things are going on and the answer to that is "yes, weird things are going on and we don't know what they are so we call them 'dark' things as placeholder names until we figure out what they are at which time we will probably give them more appropriate names".

6. Mar 23, 2015

### wolram

Why not admit the model is wrong rather than fitting in these DARK things, i think it is correct to say that Dark Matter has been searched for for 20yrs or more and we are no nearer a solution.

7. Mar 23, 2015

### julcab12

It can either be a phenomenon emerging from our incomplete understanding of dynamics (Good -- until Bullet Clusty ruin the party) , Or it can be something we can't see deviated from what we expect like normal matter behave -- still materialistic/exotic matter. We don't have material evidence except clues.

http://www.munich-iapp.de/scientific-programme/programmes-2015/dark-matter/ [Broken]

Last edited by a moderator: May 7, 2017
8. Mar 23, 2015

### Doug Huffman

Dark Matter was postulated almost 100 years ago, in 1932, by Jan Oort, and in 1933 by Fritz Zwicky for the anomalous velocities of galaxies in galactic clusters.

9. Mar 23, 2015

### wolram

10. Mar 23, 2015

### wolram

100yrs and still searching surly the trail has gone cold, or do i mean warm.

11. Mar 23, 2015

### phinds

I don't get you at all. The existence of these anomalies IS a clear statement that the models are "wrong" in that they don't tell the whole story. Perhaps I'm misunderstanding you but you seem to be just shooting at strawmen here, not objecting to anything meaningful. Do you disagree that the anomalies exist? Would you prefer that we just ignore their existence instead of giving them placeholder names?

12. Mar 23, 2015

### wolram

What i am objecting to is giving these phenomena labels at all, there are so many papers in the literature that to date mean nothing, so i am objecting to the use of any material
causing these effects, No Dark Matter No dark Energy as you say the model is wrong but we sill keep looking on the dark side.

13. Mar 23, 2015

### Doug Huffman

Many consider the Standard Model(s), the models of particle physics and cosmology, to be unfalsifiable towers of ad-hockery. They cry "vastly verified" reducing science to mere technology.

14. Mar 23, 2015

### phinds

So how do we talk about them if we don't give them names? I mean, it's real pain to say "that weird thing that we don't understand that is causing the expansion of the universe to accelerate" when we can just say "dark energy".

15. Mar 23, 2015

### Chalnoth

This is a strong understatement of the evidence available for dark matter. The anomalous rotation of galaxies is probably the weakest evidence we have for dark matter. Galaxy cluster observations are considerably more striking as evidence for dark matter. The bullet cluster is particularly striking as a recent collision separated the dark matter from the visible matter, and the gravitational lensing of the background galaxies shows most of the mass is centered on the dark matter, not the visible matter. The cosmic microwave background is almost impossible to explain without dark matter (there is clear evidence of a component of matter that feels pressure, and a component of matter that does not feel pressure, which can only be true if that matter does not interact with light, i.e., if it's dark).

As for dark energy, the evidence is much less varied, but it's by far the simplest way to reconcile the observed geometry of our universe with the observed matter (normal + dark) density of our universe. All of the simple alternatives have so far failed (e.g., it was thought for a while that the fact that our universe isn't perfectly homogeneous and isotropic could account for the discrepancy, but this explanation ultimately didn't fit the data).

16. Mar 23, 2015

### Almeisan

We have all the observed evidence.

We find the best model to fit this data. There are anomolies. We try to account for the anomolies the best way we can, calling them 'dark' and not putting a label on what causes it, just that it is there.

The alternative is to find a better model that does not have anomolies.

Or, we can just throw up our hands and say 'We have no idea. Since there are anomolies, we can't know anything at alll, eventhough it looks like our models are correct." and wait for someone to come along and come up with a model that has no anomolies.

And while we are at it, why not throw away all other models that have anomolies, like QM and SR&GR.
Remember than now we can't use the 'false models', eventhough they lead to useful things like modern el;ectronics. We don't have any models for their inner workings, as we are not allowed to use QM. So how do we know how to build computer chips?

17. Jan 26, 2016

### Chalnoth

The evidence for dark matter today is quite solid. Here's a pretty good blog post on the subject:
http://www.preposterousuniverse.com/blog/2006/08/21/dark-matter-exists/

There are lots of other pieces of evidence, but the Bullet Cluster is a nice, visual demonstration of dark matter. Though personally I feel that the CMB evidence is much stronger and harder to explain with any alternative model.

As for dark energy, that's a bit more uncertain at the current time. Back in the late 90's/early 00's, there was a flurry of work involving attempts to find alternative explanations for the observations. Currently there are only three main explanations that still fit with the evidence:
1. Dark energy.
2. Modified gravity.
3. Cosmological constant (which is largely considered a subset of option 1).

Modified gravity is largely only still in the running simply because it's a wide-open possibility. There haven't actually been any specific proposals that work well theoretically and also fit the evidence. But the theoretical work that has been done has managed to show that it's really, really hard to modify General Relativity in a self-consistent way that explains the acceleration but doesn't cause other observational problems.

Dark energy is also there because it's wide-open: there aren't any well-motivated theoretical models where some kind of dark energy is derived. There are nothing but ad-hoc models whose specific purpose is to produce dark energy. There are probably many hundreds of such models that fit with the evidence, because nearly all dark energy models are indistinguishable from a cosmological constant with certain parameter choices. Dark energy, at this point, is largely a placeholder for, "Maybe there's a cause we don't yet know?"

The only (somewhat) well-motivated theoretical model around is the cosmological constant, as it is an unavoidable component of General Relativity, one that people had for a long time assumed must be set to zero by some as-yet-undiscovered fundamental symmetry. This was largely assumed to be zero because it has to have a magnitude less than about $10^{-120}$ for the universe to not either immediately recollapse or expand so quickly that no structure forms, and theorists generally feel that such an absurdly small number is highly unlikely to occur. No such symmetry has yet been found.

Largely the theories around the cosmological constant fall into two groups:
1. The cosmological constant is dynamically determined by some physical processes, and has a tendency to relax to very small values (here's one example among many: http://arxiv.org/abs/hep-th/0311011)
2. The cosmological constant takes on more-or-less random values in different regions of the universe and the one we observe is so small because there would be nobody here to observe it otherwise. This is the case under the string theory landscape, for example.

18. Jan 27, 2016

### newjerseyrunner

Not true, cosmologists know a lot more about dark matter now than they did 100 years ago. For example: because of the bullet cluster, we now know that dark matter does not collide with itself the way that gas does. Through meticulous observation, they've also eliminated all known astronomical phenomenon: black holes, rogue planets, cold dust, neutrinos, virtual particles... Because of observations of galaxy halos, we know that it must be fairly warm. They've also been able to essentially map where it is.

I agree about dark energy, but it's fairly new. It was first discovered in the 1990s, I'm sure as time goes by, we'll get a much deeper understanding of it.

All of this is simply showing us that our physics is incomplete, Newton knew that his theory of gravity was not complete, he had a weird constant with a really weird unit attached to it.

19. Jan 27, 2016

### RUTA

20. Jan 27, 2016

### Staff: Mentor

Thread closed for moderation.