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JuanCasado
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Could anyone clarify where is gone the energy lost by CBR as the universe expands?
Thank you in advance and happy new year
Thank you in advance and happy new year
Drakkith said:I've never seen it put quite like that MFB. Can you elaborate? Are you referring to dark energy or something else?
Drakkith said:Yes but how do photons lead to expansion pressure?
phinds said:I don't think it IS now, just that it was prior to the surface of last scattering. The expansion since then has been sort of a ballistic remnant of the expansion at that time. I am NOT sure about that, though. I could be just making it up, but I think that's what I've read.
The photons don't lead to expansion pressure. The pressure that a photon gas exerts, however, gravitates (there is as much attractive gravity caused by the energy density of photons as by their pressure). This is what makes it so that a universe dominated by photons slows its expansion much more rapidly than one dominated by normal matter, and also what makes photons lose energy as the universe expands.Drakkith said:Yes but how do photons lead to expansion pressure?
Chalnoth said:The photons don't lead to expansion pressure. The pressure that a photon gas exerts, however, gravitates (there is as much attractive gravity caused by the energy density of photons as by their pressure). This is what makes it so that a universe dominated by photons slows its expansion much more rapidly than one dominated by normal matter, and also what makes photons lose energy as the universe expands.
Yes. They are more efficient at slowing the expansion than normal matter. You can see this in the second Friedmann equation:Drakkith said:So photons don't lead to an increase in expansion rate then? They slow it down?
Yes.Drakkith said:What exactly is meant by "photon pressure"? Is this the same thing as normal Radiation pressure?
Chalnoth said:One way to understand why it has this effect is to consider a somewhat different scenario:
Imagine that we have an enclosed box, and within that box is a gas of photons (you can simply imagine the box as having some temperature, which causes it to be filled with radiation). That gas of photons exerts radiation pressure on each wall of the box equal to [itex]\rho/3[/itex].
Now, what happens if we cause this box to expand in size? Well, if the box expands by a factor of [itex]a[/itex], then the photon pressure on each side of the box exerts work on the box. Because the work is in the direction of the motion of the walls of the box, this amounts to a transfer of energy from the photon gas to the walls of the box. In fact, if you calculate the energy transfer, you exactly get the loss of energy of the photon gas that we see as a redshift.
As to why this pressure leads to a faster slowdown of the expansion, well, that's a bit harder to explain. But suffice it to say that pressure is sort of a kind of energy density, and gravity responds just as well to this sort of energy density as it responds to mass energy.
bcrowell said:This is incorrect. You're comparing (1) a cosmological spacetime with (2) a system consisting of a box with photons inside. In case 1, energy isn't conserved (as explained in the FAQ linked to in #15). In case 2, energy is conserved.
Chalnoth said:One way to understand why it has this effect is to consider a somewhat different scenario:
Imagine that we have an enclosed box, and within that box is a gas of photons (you can simply imagine the box as having some temperature, which causes it to be filled with radiation). That gas of photons exerts radiation pressure on each wall of the box equal to [itex]\rho/3[/itex].
Now, what happens if we cause this box to expand in size? Well, if the box expands by a factor of [itex]a[/itex], then the photon pressure on each side of the box exerts work on the box. Because the work is in the direction of the motion of the walls of the box, this amounts to a transfer of energy from the photon gas to the walls of the box. In fact, if you calculate the energy transfer, you exactly get the loss of energy of the photon gas that we see as a redshift.
As to why this pressure leads to a faster slowdown of the expansion, well, that's a bit harder to explain. But suffice it to say that pressure is sort of a kind of energy density, and gravity responds just as well to this sort of energy density as it responds to mass energy.
It is a different system, but I think it is rather interesting that the math works out identically.bcrowell said:This is incorrect. You're comparing (1) a cosmological spacetime with (2) a system consisting of a box with photons inside. In case 1, energy isn't conserved (as explained in the FAQ linked to in #15). In case 2, energy is conserved.
It's not really an analogy. It's a slightly different system that behaves in the exact same way mathematically. This is at least suggestive that we can think of gravity as soaking up the lost energy in the expanding photon field.julcab12 said:Good analogy but for noobies like me. It would be a good thing if you mention such limitations, conditions and a brief elaboration of each component first to avoid misconceptions. :shy:
I don't think there can be anything particularly special here, at least not anything pointing to new physics.JuanCasado said:The analogy described seems reasonable and clarifying to me. However, the amount of ordinary matter within the observable universe results to be similar to the amount of energy lost by CBR since decoupling...
Just an accident by mere chance? or perhaps a kind of new coincidence problem in cosmology?
Cosmic Background Radiation (CMB) is the leftover energy from the Big Bang, which is the event that is believed to have created the universe. It is a type of electromagnetic radiation that permeates the entire universe and is one of the oldest and most abundant sources of energy in the universe.
The Cosmic Background Radiation was first discovered in 1965 by two scientists, Arno Penzias and Robert Wilson, who were studying microwave signals from space. They noticed a constant noise that seemed to be coming from all directions, and after ruling out all other sources, they concluded that it was the CMB.
The lost energy of Cosmic Background Radiation is significant because it can provide important insights into the early stages of the universe. By studying the temperature and distribution of CMB, scientists can learn more about the composition and evolution of the universe and potentially uncover new information about the Big Bang.
Scientists use specialized instruments, such as radio telescopes and satellites, to measure the CMB. These instruments are able to detect and measure the faint signals of CMB radiation from all directions in space. By analyzing this data, scientists can determine the temperature and energy of the CMB.
There are several theories that attempt to explain the lost energy of Cosmic Background Radiation. Some scientists believe that the energy has been absorbed by dark matter particles, while others suggest that it has been dispersed throughout the universe and is no longer detectable. There are also theories that propose the energy has been converted into other forms, such as gravitational waves or neutrinos.