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alialice
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How can inflation explain the particle horizon's problem?
Inflation solves this problem by allowing the universe to expand slowly at first and reach a specific temperature via causal contact, and then undergo an enormous expansion.
Mark M said:The horizon problem refers to the fact that the universe is the same temperature everywhere, yet it should have expanded far too fast for the temperature to balance out. Inflation solves this problem by allowing the universe to expand slowly at first and reach a specific temperature, and then undergo an enormous expansion.
Naty1 said:Is THAT the 'problem' to which you refer?
alialice said:How can inflation explain the particle horizon's problem?
Naty1 said:yes:
alialice:
But I am not sure there is a 'particle horizon problem"
As a consequence of this inflationary expansion, all of the observable universe originated in a small causally connected region. Inflation answers why does the universe appear flat, homogeneous, and isotropic...
Is THAT the 'problem' to which you refer?
It has been said that the Hubble length decreases during inflation (page 9).
...The quantum fluctuations generated during the inflationary phase — which act as seeds of structure formation in the universe— can be characterized by their physical wavelength. Consider a perturbation at some given wavelength scale which is stretched with the expansion of the universe as λ ∝ a(t). During the inflationary phase, the Hubble radius remains constant while the wavelength increases...,
Naty1 said:My question:
So the first two sentences are ok...but can someone explain the underlying logic from which I can understand why the 'Hubble radius remains constant'...yet wavelength is stretched as the scale factor [a] evolves?
and the answer is post #10 here:
https://www.physicsforums.com/showthread.php?p=3984153#post3984153
There is some other discussion in that thread that may also be of interest to you.
alialice said:I know that during the inflationary phase, H=Lambda/3, where Lambda is the cosmological constant, and the expansion is exponential. So H is constant.
But in Liddle it is said:
"During inflation
[itex]\frac{d\left( H^{-1} /a\right)}{dt}<0 [/itex]
The Hubble length, as measured in comoving coordinates, decreases during inflation.
alialice said:At any other time the comoving Hubble length increases. This is the key property of inflation;
Inflation is a hypothetical period in the early universe where the universe expanded rapidly, causing it to grow exponentially in size. This is believed to have occurred within the first fractions of a second after the Big Bang.
Inflation is thought to have caused the universe to expand so quickly that any irregularities or variations in density were smoothed out, resulting in a more homogeneous and isotropic universe.
The particle horizon is the maximum distance that light can have traveled in the universe since the Big Bang. Inflation is thought to have expanded the universe beyond the particle horizon, allowing for regions of the universe that were previously causally disconnected to become connected.
The cosmic microwave background radiation, which is leftover radiation from the Big Bang, shows a high degree of homogeneity and isotropy, as predicted by the theory of inflation. Inflation also provides a solution to the horizon problem, which is the question of how distant parts of the universe could have the same temperature when they were never in contact.
Yes, there are alternative theories to inflation, such as the ekpyrotic universe model and the cyclic universe model. These theories propose that the universe goes through cycles of contraction and expansion, rather than a single period of rapid inflation.