Table of Contents

## What is conformal time?

Rather, the conformal time is the amount of time it would take a photon to travel from where we are located to the furthest observable distance, provided the universe ceased expanding.

### What is matter radiation equality?

The instant of matter–radiation equality divides the two main epochs of the Universe’s evolution, radiation domination and matter domination. It took place when the Universe was approximately 50 000 years …

#### What is Hubble time in astronomy?

The time required for the Universe to expand to its present size, assuming that the Hubble constant has remained unchanged since the Big Bang. It is defined as the reciprocal of the Hubble constant, 1/H0. For a Hubble constant of 71 km/s/Mpc, as given by current measurements, the Hubble time is around 14 billion years.

**What is the difference between comoving and proper coordinates?**

Comoving distance and proper distance are defined to be equal at the present time. At other times, the Universe’s expansion results in the proper distance changing, while the comoving distance remains constant.

**What is comoving volume?**

The comoving volume VC is the volume measure in which number densities of non-evolving objects locked into Hubble flow are constant with redshift. It is the proper volume times three factors of the relative scale factor now to then, or (1 + z)3.

## What is the redshift for matter-radiation equality?

We can calculate the redshift of matter-radiation equality by using the scaling of the energy density of radiation and matter: ρmatter=ρ0matter(1+z)3, ρradiation=ρ0radiation(1+z)4, where ρ0 denotes the current enegy density.

### What is Omega radiation?

Omega radiation is a type of radiation. In 2376, the con artist Mobar, pretending to be “Tuvok”, informed two Telsian miners that their bolomite could be used for omega radiation therapy.

#### Why is lookback time important?

Lookback time is what makes the subfield of galaxy evolution possible, the ability to study the changes in galaxies with time by observing them at various distances means equals different epochs.

**What is lookback time in cosmology?**

The lookback time tL to an object is the difference between the age t0 of the Universe now (at observation) and the age te of the Universe at the time the photons were emitted (according to the object).

**How is Hubble parameter calculated?**

The Hubble Constant can be stated as a simple mathematical expression, Ho = v/d, where v is the galaxy’s radial outward velocity (in other words, motion along our line-of-sight), d is the galaxy’s distance from earth, and Ho is the current value of the Hubble Constant.

## What is Hubble distance?

This is the distance of an object based on the Hubble flow. It is the speed of light, c times the Hubble time, tH (or equivalently c divided by the Hubble constant H0) and is called the Hubble distance DH.

### What is age of universe at matter radiation equality?

About 50000 years after the Big Bang, the density of matter was now higher than the density of radiation. The universe had reached the epoch of matter-radiation equality. R(t) =1+ z, meaning that the universe has expanded with a factor of 1100 since the recombination period.

#### What does Hubble time represent?

**How does Hubble calculate time?**

You can actually calculate an estimate for the age of the Universe from Hubble’s Law….Well, 300 miles / 60 mph = 5 hours.

- So, the time it has taken for the galaxies to reach their current separations is t=D/v .
- But, from Hubble’s Law, we know that v=H0D .
- So, t=D/v=D/(H0×D)=1/H0 .
- The best estimate for H0=73km/s/Mpc .