# What is the rate of cosmic expansion?

Relevance
• 7 months ago

A recent calculation of rate of expansion of the universe is 70.0 kilometers per second per megaparsec.

The “per megaparsec” is an important part of this rate. (A parsec is a unit of length equal to about 3.26 light years, or 30,856,774,879,000 km.)

~ It means that a galaxy that is one million parsecs away from us is receding from us at only 70 kilometers per second because of the expansion of the universe.

~ A galaxy that is TWO million parsecs away would be receding from us at twice that velocity; 140 kilometers per second.

~ A VERY distant galaxy that is over 4150 million parsecs (over 13.5 billion light years) away from us is receding faster than light speed and we can never receive light from that area.

Those galaxies would be receding from us because space is expanding. They cannot move "through" space faster than light speed; that is impossible.

• orpheus_sword
Lv 5
7 months agoReport

Donut Tim is mostly correct. The Hubble parameter at today is 70 km/s/Mpc, but it was a different value in the past based on the proportion of dark matter, dark energy,... The math isn't quite right, since this linear scaling between distance and Hubble parameter doesnt hold on cosmological scales.

• 7 months ago

The rate of expansion of space (not the Universe) is different in all places, also depending from where you look. The expansion is not constant, although the "Hubble Constant" would suggest otherwise. The only thing we do know is that at the edge of visible space the accumulation of expanding space between us and a distance object exceeds the speed of light. That value lies around 46.5 billion light years. For an object at that distance the rate is thus 300,000km/h.

• Larry
Lv 7
7 months ago

These gravitational waves can work as a sort of standard sirens to measure the expansion rate of the universe. Abbot et al. 2017 measured the Hubble constant value to be approximately 70 kilometers per second per megaparsec.

• 7 months ago

74 kilometers per second PER MEGAPARSEC. Do the math to cancel out the per megaparsec term to get a speed. Its something like 10^-14 inches/centimeters per second. I am not doing to look up the text file on my PC at 5:40 am CDT. I worked it out more than 7 YEARS ago.

The Hubble constant is NOT "constant."

I may look it up in several hours and edit this answer then.