Jeans' length Flashcards
What is the Jeans’ length (λJ)?
The length at which the gravitational attraction and gas pressure are balanced (right hand side of matter of growth of fluctuation eq is equal to zero)
What happens if λ»_space; λJ?
Gravity wins (big things)
What happens if λ«_space;λJ?
Pressure wins (small things)
If π is assumed to be 3, what is the relationship between 𝝀J and 𝝀H?
they are close in size due to free streaming of gravitational dominant photons and neutrinos
Why are there no pressure effects above horizon?
As the horizon is linked to speed of light
What is is the first term in the brackets (4πGρ) represent in the fluctuations formula?
Self-gravity
What is the second term in the brackets (k^2c^2/a^2) represent in the fluctuations formula?
pressure
When can cold dark matter fluctuations grow on all scales?
After matter-radiation equality
When do baryons grow on scales smaller than horizon at matter radiation equality?
At recombination
What happens to baryonic matter after recombination?
It can fall into the gravitational potential wells that the cold dark matter has been forming since the matter radiation equality
What happens to the baryon fluctuations after recombination?
The baryon fluctuations can be significantly larger than the fluctuations in the CMB
After recombination, what does baryon pressure effect?
Small scale structures only as Jean’s length is small (size of globular cluster)
What does the Jean’s length set?
The split between wavelengths where fluctuations can grow under the influence of their own self gravity and those where pressure dominates and no growth is possible
When do fluctuations stop growing?
Once universe behaves as if it were empty
