Mass-dominated vs. radiation-dominated.

  • Thread starter borvis
  • Start date
In summary, the universe can be either mass-dominated or radiation-dominated, depending on whether the energy-density is dominated by mass or radiation. This is determined by the expansion factor and the energy density of each component. Before the equivalence epoch, the radiation energy is greater and it is considered the "radiation-dominated era." After the equivalence epoch, the matter energy is greater and it is considered the "matter-dominated era." Hubble's law, which relates the expansion factor to the speed of objects moving away from each other in space, is a local observation and does not fully capture the complexity of the universe's expansion.
  • #1
borvis
7
0
I have a simple question:
What does it mean that the universe is mass-dominated or radiation-dominated?
I can't find the answer anywhere on the internet. But i kind of think that it means that the energy-density of the universe is either dominated by the energy from mass or radiation. Is this correct? Thanks alot.
 
Space news on Phys.org
  • #2
Yes, that's correct.
 
  • #3
Thanks!
 
  • #4
If you call [tex]a[/tex] the expansion factor, the energy density of the radiation goes as [tex]\rho_{rad}\propto a^{-4}[/tex], while for the matter you have [tex]\rho_{mat}\propto a^{-3}[/tex]. If you plot logarithmically these quantities, you will see that at a certain [tex]a_{eq}[/tex] the two densities will be equal: there it is the equivalence epoch. Before equivalence the radiation energy is greater that the matter energy, so you talk about "radiation-dominated era". For [tex]a>a_{eq}[/tex], you have the matter-dominated era.
 
  • #5
And this means because of Hubbles law, v=Hr, you can say that a [tex]\propto[/tex] r? Since the expansion factor is proportional to the speed that objects move from each other in space?
 
  • #6
Not exactly, since in cosmology, H(t) is defined as (da/dt)/a (sort of like your v/r).

Hubble's law is really just a local observation of the universe expanding.
 

What is the difference between mass-dominated and radiation-dominated?

In cosmology, the terms mass-dominated and radiation-dominated refer to two different periods in the early universe. During the mass-dominated era, the universe was dominated by matter, specifically dark matter, which had a greater influence on the expansion of the universe. In the radiation-dominated era, the universe was dominated by radiation, which had a greater influence on the expansion.

How do these two eras affect the expansion of the universe?

The difference in dominance between matter and radiation has a significant impact on the expansion of the universe. During the mass-dominated era, the expansion of the universe was slower because dark matter has a weaker gravitational pull compared to radiation. However, during the radiation-dominated era, the expansion was much faster due to the high energy and pressure of radiation.

Which era came first in the history of the universe?

The radiation-dominated era came first in the history of the universe. This era lasted from the initial moments of the Big Bang until about 50,000 years after. It was during this time that the universe underwent rapid expansion, known as cosmic inflation, which was driven by the high energy and pressure of radiation.

What caused the transition from the radiation-dominated to the mass-dominated era?

The transition from the radiation-dominated to the mass-dominated era was caused by a process known as recombination. This is when the universe cooled enough for protons and electrons to combine and form neutral atoms, which caused the universe to become transparent to radiation. This marked the end of the radiation-dominated era and the beginning of the mass-dominated era.

How do scientists study and observe these eras?

Scientists study and observe the mass-dominated and radiation-dominated eras through various methods, including analyzing the cosmic microwave background radiation, studying the abundances of light elements, and observing the large-scale structure of the universe. These methods allow scientists to learn more about the early universe and the different eras it went through.

Similar threads

  • Cosmology
Replies
4
Views
194
Replies
5
Views
875
Replies
8
Views
1K
  • Cosmology
Replies
7
Views
2K
  • Cosmology
Replies
11
Views
1K
Replies
6
Views
1K
Replies
1
Views
770
  • Cosmology
Replies
2
Views
964
Replies
19
Views
415
  • Cosmology
Replies
22
Views
4K
Back
Top