I Do dark matter and dark energy have an effect on the red shift?

[timmdeeg]

Yes, my terminology was sloppy

I was confused myself when I came across this sloppyness in a pop science cosmology book by Prof. Harald Lesch some time ago. So from this point of view you are in good company.

 

[kimbyd]

I think your original response was correct, actually.
After all, the rate of expansion (i.e. the Hubble parameter) falls as 1/t in an empty universe, and falls faster than that in a universe with matter in it. So, given two universes with identical initial expansion rates, but different matter contents, the one with less matter in it will result in higher H after time t from the start of the expansion.

I'm really not sure this kind of comparison is meaningful. The problem is that the initial conditions aren't fixed, and there's no good way to fix them. Certainly, a universe will more matter will have a lower expansion after a time t than a universe with more matter, provided that they both start with the same initial expansion rate.

But it's not at all clear that they would have the same initial expansion rate. Less dark matter means different high-energy laws of physics, which means lots of things would be different.

 

[Arman777]

But it's not at all clear that they would have the same initial expansion rate. Less dark matter means different high-energy laws of physics, which means lots of things would be different.

What do you mean by same initial expansion rate?

Also, Is it really makes that difference? Without matter and dark matter we can model universe (empty space with lambda case). For example, If we had less/more baryonic matter the physics rules would be the same. So why would it change for less dark matter?

 

[kimbyd]

What do you mean by same initial expansion rate?

Also, Is it really makes that difference? Without matter and dark matter we can model universe (empty space with lambda case). For example, If we had less/more baryonic matter the physics rules would be the same. So why would it change for less dark matter?

The amount of dark matter we have is likely due to the nature of some kind of high-energy symmetry breaking event in the early universe. A change in that event which would have resulted in less dark matter would likely have changed many other things about our observable universe, which makes doing the thought experiment very, very tricky.

What we can say is that matter (whether normal or dark) tends to slow the rate of expansion. Radiation tends to slow the rate of expansion faster. A cosmological constant makes the rate of expansion tend towards a constant rate.

 

[Arman777]

The amount of dark matter we have is likely due to the nature of some kind of high-energy symmetry breaking event in the early universe. A change in that event which would have resulted in less dark matter would likely have changed many other things about our observable universe, which makes doing the thought experiment very, very tricky.

What we can say is that matter (whether normal or dark) tends to slow the rate of expansion. Radiation tends to slow the rate of expansion faster. A cosmological constant makes the rate of expansion tend towards a constant rate.

Hmm I understand it I guess , thanks. But what about "initial expansion rate". What it means ?

 

[timmdeeg]

I'm really not sure this kind of comparison is meaningful. The problem is that the initial conditions aren't fixed, and there's no good way to fix them. Certainly, a universe will more matter will have a lower expansion after a time t than a universe with more matter, provided that they both start with the same initial expansion rate.

But it's not at all clear that they would have the same initial expansion rate. Less dark matter means different high-energy laws of physics, which means lots of things would be different.

It seems to me that the same initial value of $H$ for both scenarios (more and less matter density) is possible by balancing a lower $\rho$ with a higher $\Lambda$. So if we compare our universe like it is with another one having a lower matter density while $\Lambda$ remains the same then if I see it correctly $H$ should be different in both cases at any finite time and should approach the same value asymptotically after infinite time.

 

[kimbyd]

Hmm I understand it I guess , thanks. But what about "initial expansion rate". What it means ?

That's the expansion rate at a specific time in the past defined as "initial".

 

[Arman777]

That's the expansion rate at a specific time in the past defined as "initial".

Hmm I see

 

[Hendrik Boom]

This is true for dark matter, but not for dark energy. Dark matter, like ordinary matter, causes light rays to converge, hence the phenomenon of gravitational lensing.

Dark energy, if it "clumped" as matter does, would cause light rays to diverge; I suppose one could call the phenomenon this would produce, if it occurred on a small scale, "gravitational anti-lensing". However, since the density of dark energy is uniform throughout the universe (unlike the density of dark matter and ordinary matter, both of which clump, though the former does so to a lesser extent than the latter), it has no effect on light rays at all, since any such effect requires a spatial variation in density.

How can we know this much about dark energy? Are there relevant observations?

 

[Arman777]

How can we know this much about dark energy? Are there relevant observations?

If we assume dark energy is an instance property of the space-time (or another words lambda (Λ)). It's easy to think like that.

 

[kimbyd]

How can we know this much about dark energy? Are there relevant observations?

The short version is that it is impossible for dark energy to both clump and also result in an accelerated expansion.

 

[ohwilleke]

How can we know this much about dark energy? Are there relevant observations?

Dark energy is inferred based upon the expansion rate of the universe and changes in the expansion rate as measured with astronomy observations such as patterns of red shift in stars and the cosmic background radiation and neutrino merger data.