Why the Universe is Expanding?

Now we know that our universe is expanding at an accelerating rate. Using standard candles with known intrinsic brightness, the expansion of the universe has been measured using redshift to derive Hubble’s Constant: H0 = 67.15 ± 1.2 (km/s)/Mpc. For every million parsecs of distance from the observer, the rate of expansion increases by about 67 kilometres per second.

It is interesting to know that what force is causing this expansion. If the universe is expanding, there must be any type of force causing this expansion.

How we know that universe is expanding?

Our modern picture of universe dates back to only 1924, the American astronomer Edwin Hubble determined the distances between galaxies by studying luminosity and spectra of the lights coming from these galaxies. Further in the 1920s, when astronomers began to look at the spectra of stars in other galaxies they found something most peculiar: there were the same characteristic sets of missing colors as for the stars in our own galaxy, but they were all red-shifted (when the source producing light moves away from us, its wavelength increases and this gives us appearance of red-shift.)

Hubble spent his time cataloging their distances and observing their spectra. He was able to show how fast all galaxies are moving far from each other. At that time most people expected the galaxies to be moving around quite randomly, so expected to find as many blue-shifted spectra as red-shifted ones. It was quite surprising, therefore, to find that most galaxies appeared red-shifted: nearly all were moving away from us!
Even the size of a galaxy’s redshift is not random but is directly proportional to the galaxy’s distance form us.
This mean that the universe could not be static, but is in fact expanding; the distance between the different galaxies is growing all the time.

Why the Universe is Expanding?

Now we know that the universe is expanding, but why and how?
So up until about 15 years ago, the answer was momentum. The idea was that the Universe received all the energy it needed for its expansion in the first few moments after the Big Bang and this enormous energy produced a huge amount of force and thus great momentum causing the universe to expand.

Gravitation is an attractive force deaccelerating the rate of expansion of universe. Would the mutual gravity of all the objects in the Universe cause it to slow to a halt at some point in the distant future, or maybe even collapse in on itself, leading to a Big Crunch? Or just clump up in piles and stay on the couch all summer because it’s maybe a little lazy and isn’t ready to start going back to the gym yet?

In 1999, astronomers discovered something completely unexpected… Dark Energy. As they were doing their observations to figure out exactly how the Universe would coast to a stop, they discovered that it’s actually speeding up. It’s as if that bullet is actually a rocket and it’s accelerating.
Now it appears that the Universe will not only expand forever, but the speed of its expansion will continue to accelerate faster and faster. So what’s causing this expansion? Currently, we believe it’s mostly momentum left over from the Big Bang, and the force of dark energy will be accelerating this expansion. Forever.