Measuring the universe

The Universe's expansion and origins can be understood through cosmic microwave background radiation (CMBR), redshift measurements, and the Hubble constant. These concepts help us determine how fast galaxies are moving away, how far they are, and even estimate the age of the Universe.

One light year

One light-year is equal to approximately 9.5 × 10¹⁵ metres (9.5 trillion kilometers). This unit is used to express the distance between stars and galaxies.

Cosmic microwave background radiation

CMBR is microwave radiation that exists everywhere in space and is one of the most important pieces of evidence for the Big Bang Theory.

Origin of CMBR

• Shortly after the Big Bang, the Universe was extremely hot and dense, filled with high-energy gamma radiation.

• As the Universe expanded, this radiation stretched due to redshift, shifting into the microwave region of the electromagnetic spectrum.

• Today, CMBR is observed uniformly in all directions, meaning it surrounds us everywhere in space.

Example: Satellites like the COBE and Planck spacecraft have mapped CMBR, showing a nearly uniform background radiation with slight variations that hint at the early structure of the Universe.

Redshift and galaxy movement

The speed at which a galaxy moves away from Earth is found by measuring redshift, which is the increase in wavelength of the galaxy’s starlight.

Finding Galaxy Speed from Redshift

• As a galaxy moves away, the wavelength of light increases, shifting toward the red part of the spectrum.

• The amount of redshift can be used to determine the speed at which the galaxy is moving.

Example: By analyzing spectral lines from galaxies, astronomers measure how much their wavelengths have stretched.

Determining distances of far galaxies

• The distance d to a far galaxy can be estimated by measuring the brightness of a supernova in that galaxy.

• Certain types of supernovae (like Type Ia supernovae) always have the same peak brightness, allowing scientists to compare how dim they appear to estimate distance.

Example: If two identical supernovae occur, but one appears dimmer, it must be farther away.

• The Hubble constant (H₀) is the ratio of the speed at which the galaxy is moving away from the Earth to its distance from the Earth.

• Hubble’s Law:

where:

• H0​ = Hubble constant

• v = speed of galaxy moving away (light years/s or km/s)

• d = distance of the galaxy (light years or km)

Current estimate of H0​:

H0 ≈ 2.2 × 10-18 per second

Estimating the age of the universe

We can estimate the age of the Universe using: