6 Expansion of the Universe Free Online FlashCards

What does cosmic expansion mean?

Cosmic expansion is the growth over time of average distances between galaxies that are not gravitationally bound to one another.

Do galaxies, solar systems, and atoms expand with the universe?

No. Gravity and other forces hold together systems such as galaxies, solar systems, and atoms, so they do not simply stretch with cosmic expansion.

Is Earth the center of cosmic expansion?

No. An observer in any typical galaxy sees distant galaxies receding on average; Earth is not the center of cosmic expansion.

What does the scale factor $a$ describe?

The scale factor, usually written as $a$, describes the universe’s changing size. It is conventionally set to $a=1$ today and was smaller in the past.

How does cosmic expansion produce cosmological redshift?

Cosmological redshift occurs because light traveling through expanding space is stretched, increasing its wavelength.

How are redshift $z$ and emission scale factor $a$ related?

The relationship is $1+z=\frac{1}{a}$, where $a$ is the scale factor when the light was emitted.

What redshift and wavelength change result from emission at $a=\frac{1}{3}$?

If light was emitted when $a=\frac{1}{3}$, then $z=2$, and its wavelength is stretched to three times its original length.

What does Hubble’s law relate?

For relatively nearby galaxies, Hubble’s law says average recession speed is approximately proportional to distance: $v\approx H_0d$.

What does the Hubble constant $H_0$ represent?

$H_0$ is the Hubble parameter’s value today and describes the present-day expansion rate. It is commonly expressed in kilometers per second per megaparsec $km/s/Mpc$.

What is the Hubble tension?

It is a discrepancy between some modern estimates of the expansion rate that do not agree within their stated uncertainties.

How can distant regions separate faster than light without violating local motion limits?

In general relativity, sufficiently distant regions can separate faster than light without locally moving through space faster than light.

What does the Big Bang describe?

The Big Bang describes space itself expanding from an early, hot, dense state about 13.8 billion years ago; it was not an explosion from one location into pre-existing empty space.