# Without Dark Matter and Dark Energy

1. Aug 22, 2010

### cbd1

What would a cosmological model without dark matter and dark energy look like?

This requires imagining that the observational evidence for dark matter and dark energy are explainable by other means. In other words, absent of the idea that the universe is filled with 25%DM and 70%DE, but rather having 100% of the the mass in the universe being what is observable (what we now see as only 5% of the mass), what kind of model would we arrive upon?

To further clarify, if it did not appear that the universe were accelerating in expansion, and it did not appear that there is more mass in galaxies than the normal matter, but rather that regular matter is all there is, combined with the current rate of expansion, would the universe have to be open? What other problems would occur having normal matter and energy be all there is in the universe? Again, ignoring the experimental data saying that the universe is accelerating and there is more mass, what would the problems be in the universe without DM and DE?

2. Aug 22, 2010

### Chronos

Inconsistent with current observational evidence. Without DM, galactic rotation curves, clusters and the bullet cluster would be inexplicable. Without DE, the supernova legacy study would not make sense.

3. Aug 22, 2010

Staff Emeritus
Inconsistent with observation. I'm not sure it's possible to go logically further than that.

4. Aug 22, 2010

Staff Emeritus
Beat me by a microsecond!

5. Aug 22, 2010

### cbd1

I thought I made it explicitly clear that this question is theoretical, and would require disregarding these exact experiments.

So, aside from these experiments not making sense without some "dark matter" (which is a guess for an experimental result we don't understand anyway) and "dark energy" (which is also a guess for an experimental result we don't understand anyway), what would the implications be for the rest of cosmology?

I presume it would not alter Big Bang theory, or the predicted age of the universe, or anything else really except for the noted experiments. There would be a problem with the Lambda-CDM model, but only really on the content of the universe part. I can also see a problem with the attempt to *make* a result we want in order to make the universe flat, to agree with the CMB evidence that it is. But, what exactly would be the problems for this particular problem. Could the universe be flat without the 25% and 70% extrapolations of DM and DE?

6. Aug 22, 2010

### nicksauce

Sure we could make a flat universe just out of baryons, just by putting in 20x more baryons. But there are plenty of problems. If we keep Ho around the same, the universe would only be about 9 Gyr old instead of 14 Gyr old, which would disagree with globular cluster ages. Constraints placed by BBN would be violated. The CMB would look like this [attachment 1] instead of this [attachment 2]. (Source: CMBFAST online tool http://lambda.gsfc.nasa.gov/toolbox/tb_cmbfast_form.cfm [Broken])

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7. Aug 22, 2010

### cbd1

It seems that there would not need to be that much extra baryons if there were no negative energy from dark energy and no dark matter... remember, dark energy must be set to zero.

Can you explain why this would make the big bang 9 billion years ago instead of 14 billion years ago?
And I am sorry, but there is no way that the CMB is just that simple.

What BBN constraints and why must H0 remain constant?

Also, what is the '5th dimension' mentioned in this calculator?

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8. Aug 22, 2010

### nicksauce

I don't know why you think dark energy has negative energy. It is most certainly positive energy. Maybe you're thinking of negative pressure.

The formula for the age of the universe, which comes from a basic analysis of the Friedmann equations, is

$$T=\frac{1}{H_0}\int\frac{da}{a\sqrt{\Omega_{\Lambda}+\Omega_{m}a^{-3}}}$$

If you put in the numbers it just happens to give 9Gyr instead of 14Gyr. In fact, one of the original reasons people wanted to introduce dark energy was to make the universe old enough to account for the ages of some globular clusters.

It's your own prerogative to disbelieve these CMB graphs, but you're free to try to reproduce them yourself. Keep in mind that CMBFAST is a well-respected program that professionals cosmologists use all the time. I'm not sure what the "5th dimension is", and I would ignore it.

I don't have my cosmology books on hand, but I'm pretty sure BBN puts some fairly strong constraints on what the photon-baryon ratio has to be. Certainly strong enough to prevent you from increasing the number of baryons by a factor of 20.

I suppose if you want to make the universe flat without DE&DM, you could shrink the Hubble constant by a factor of sqrt20, down from 72 to 16. Of course this is highly inconsistent with all observations, but I guess no more so than increasing the number of baryons by a factor of 20. If that was the case, the universe would now be a rather absurd 41 Gyr old.

9. Aug 23, 2010

### Chronos

Science is like a jigsaw puzzle without a picture on the box. We align the pieces according to how they fit and allow the picture to emerge on its own. At the moment, a large number of pieces fit in a way suggesting it looks like the LCDM model - of which DM and DE are firmly entrenched. New observational evidence is necessary to remove these pieces from the puzzle - or a theory that better fits.

Last edited: Aug 23, 2010
10. Aug 23, 2010

Staff Emeritus
That's not "theoretical".

If you want to be doing science, you have to be consistent in your assumptions. Just because you can create a grammatical question doesn't mean it's a valid question, even "theoretically". Theoretically, what is a square circle?

11. Aug 23, 2010

### cbd1

Let me rephrase what I am asking from a different view, as to hopefully get some more adequate replies:

Suppose the observations of the supernovae surveys were explained by some unforeseen observational error, and the universe is really not expanding at an accelerating rate.
And, suppose that rotation curves of galaxies and gravitational lensing of clusters could be described by some aspect of spacetime other than suggesting there is more mass in the systems.

Then, we could drop the dark energy and dark matter ideas, showing that they are not real things. How, then, would this affect the rest of cosmology?

12. Aug 23, 2010

### cepheid

Staff Emeritus
To answer the second part in bold: we have NO way of knowing how it would affect cosmology because your supposition (the first statement in bold) requires some unspecified modification to (or elimination of) General Relativity, which is the basis of all current cosmological models. Some change to "some aspect" of space-time does leave us with enough information to figure out what the result would be.

13. Aug 23, 2010

### cesiumfrog

I think this is an interesting thread, because while most people know the basic arguments for these theoretical concepts, I for one could learn more about the independent supporting evidence.

Are the ages of globular clusters simply taken to be the ages of the oldest stars within (based on stellar evolution models)? And these presently produce figures merely a few percent younger than the universe itself? According to WP, until recently improved measurements of the properties of these stars and of the Hubble constant, those ages paradoxically appeared to be the other way around. This seems to imply that globular cluster ages are consistent with the age of the universe one would predict naively from extrapolating back the Hubble flow. So what you're saying is that if GR is correct and FRW is sufficiently applicable, then (neglecting acceleration by dark energy and deceleration by dark matter) the intergalactic expansion would still have been slowing dramatically (that is, especially without dark energy, the relativistic correction would imply a younger universe than the naive constant-rate extrapolation, making the age of those stars in the clusters inexplicable)? I take it the globular cluster evidence would also be consistent with a model in which there was no dark energy nor dark matter and there was also significantly less normal matter than is actually visible?

In what way does nucleosynthesis fit better to the model containing unknown additional species of particles?

Is there a simple explanation of how dark energy and additional matter each affect the CMB anisotropy?

14. Aug 25, 2010

### shomas

I have posted a question in another thread https://www.physicsforums.com/showthread.php?t=424244 . It asks if CMB radiation pressure differences that arise from motion with respect to the CMB reference frame, when considered on the scale of galaxies and billions of years, do away with the need for dark matter theories.

15. Dec 12, 2010

### NickMarkov

Can’t we assume that galactic disks are nearly two-dimensional objects? The two-dimensional Newtonian potential is Ln(r), and it produces constant rotational velocities v(r) = const, which would agree with the observations.

16. Dec 12, 2010

### Rick88

an example of a model that doesn't take into account dark matter and dark energy is the Einstein-de Sitter Universe.

In such model, the size of the Universe goes as t2/3. (And the Universe is assumed to be flat, which seems to be the case anyway)

It was thought to be quite accurate until observations showing the expansion of the universe accelerating were made.
Still, it is believed to be correct from the moment the Universe became transparent until the recent observations.

R.