I How Does Baryonic Matter Alone Affect the Universe’s Structure?

[Ranku]

We know that observed helium abundance constrains the amount of baryonic matter. Leaving that aside, is there any fundamental constraint on how much baryonic matter can exist in the universe, without changing the observed structures or physics of the universe? If there were only baryonic matter and dark energy constituting a flat universe, would the universe look any different?

 

[MikeeMiracle]

My laymen response: I would say yes there must be a limit. Einstein showed us that matter is just compressed energy and other laws state that energy cannot be destroyed, only converted into other forms. There is only so much energy in the universe that you can compress to make a certain amount of baryonic matter. I guess the limit is the amount of energy that was present at the universe's creation.

As I said its a laymans responce (my personal intuition,) so interested in hearing others responses on this.

 

[phinds]

Well, if there were no dark matter and no electrons, neutrinos, etc, it seems clear that the universe would be devoid of life and very likely devoid of large scale structures as well.

 

[George Jones]

no electrons

Electrons are part of baryonic matter ... to a cosmologist!

 

[phinds]

Electrons are part of baryonic matter ... to a cosmologist!

OK, but they ARE fermions and his question was specific.

 

[George Jones]

OK, but they ARE fermions and his question was specific.

A quote from page 19 of the nice text "Introduction to Cosmology" by Ryden: "Electrons (e-) are examples of leptons, a class of elemenatary particles that are not made of quarks ... The mass of of an electron is much smaller than that of a neutron or proton ... For this reason, the component of the universe made up of ions, atoms, and molecules is generally referred to as baryonic matter"

And I think it is to this component of matter to which @Ranku refers (i.e., I don't think Ranku means to exclude normal atoms and molecules with trheir accompanying electrons) when Ranku writes

We know that observed helium abundance constrains the amount of baryonic matter. Leaving that aside, is there any fundamental constraint on how much baryonic matter can exist in the universe, without changing the observed structures or physics of the universe? If there were only baryonic matter and dark energy constituting a flat universe, would the universe look any different?

 

[phinds]

And I think it is to this component of matter to which @Ranku refers (i.e., I don't think Ranku means to exclude normal atoms and molecules with trheir accompanying electrons) when Ranku writes

Fair enough.

 

[Buzz Bloom]

We know that observed helium abundance constrains the amount of baryonic matter. Leaving that aside, is there any fundamental constraint on how much baryonic matter can exist in the universe, without changing the observed structures or physics of the universe? If there were only baryonic matter and dark energy constituting a flat universe, would the universe look any different?

H i Ranku:

I have the impression that the other responders did not understand what you were asking. As I understand your question, you want to know theoretically what would be the nature of a hypothetical flat universe without any dark energy and without any radiation. Are you familiar with the "first" Friedman equation?

https://en.wikipedia.org/wiki/Friedmann_equations​

In this article the equation appears just above the heading "Useful solutions".
Your question assumes that the only term on the RHS is the one with Ω_0,M which would then have the constant value 1.

If this is what you have in mind, I can try to explain the implications. I think the assumption that Ω_0,R=0 has the most serious physics implication implying no photons and no electromagnetic interactions. On the other hand, if you assume that currently 1>>Ω_0,R>0, which is the present reality, then that would not require new physics, and the best model of the universe would be almost the same as it is now, and the same as it was before the dark energy concept became relevant.

Regards,
Buzz

 

[George Jones]

I have the impression that the other responders did not understand what you were asking.

I don't think that anything I wrote above indicates that I misunderstood @Ranku 's question (even though I ddn't answer the question.

Your question assumes that the only term on the RHS is the one with Ω_0,M which would then have the constant value 1.

If there were only baryonic matter and dark energy constituting a flat universe, would the universe look any different?

This has implications for the observed Cosmic Microwave anisotropy data. See, e.g.,

http://background.uchicago.edu/~whu/intermediate/driving2.html

 

[ohwilleke]

If there were only baryonic matter and dark energy constituting a flat universe, would the universe look any different?

I think another way to posing at least this part of the question is "What are the observable phenomenological effects of dark matter phenomena?"

This would change the cosmic background radiation signature, as previously noted, although nobody but a few astronomers and physicists would notice, and even they wouldn't care very much except that it would serve as evidence for the absence of dark matter in the universe.

More importantly, it would really materially change galaxy dynamics, impeding galaxy formation and making those galaxies that did form smaller. In a world without phenomena attributed to dark matter, for example, the fringes of spiral galaxies would spin off into space, the large scale structure of matter distribution in the universe would be much less strongly organized in a filamentary manner, and galaxy clusters wouldn't hold together as well. Put another way, the distribution of matter would be more homogeneous at smaller distance scales than it is today.

As inhabitants of a system at the edge of a spiral galaxy that would not have been within the gravitational bounds of the galaxy without dark matter phenomena, this would probably leave our system a stray in deep interstellar space, assuming that it even formed in the first place given weaker cluster dynamics.

I use that phrase "dark matter phenomena" rather than "dark matter", since it is possible that all or most of the phenomena attributed to dark matter could instead be due to discrepancies between general relativity as conventionally applied to galaxy and larger scale systems, and the actual weak field behavior of gravity in real life. But, the mechanism causing these phenomena is basically irrelevant to the consequences of these phenomena being absent.

 

[Vanadium 50]

We have an unclear question, and three very different ideas about what the OP must have meant. Why not let him clarify?