Expanding Universe Theory
It’s very difficult to be offensive when the fundamental concepts of space, time or matter —all born from a big bang— still throw even the greatest luminaries of science into bewilderment, and they rightly call everything and everyone into question.
Anyway, to resolve some doubts about matter, its creation and its conservation, you can read something about nucleosynthesis and Antoine Lavoisier’s law. In this post, however, I want to focus on the expanding universe theory. Don’t worry, I’m not an astrophysicist, so this will just be a little, amazing story (thanks to the protagonists, not for my grammar).
Theoretical physicists may bring back the stationary universe theory (no Big Bang and no inflation as we know them) and think that there is no expansion —which is replaced by an increase in mass of everything as the universe ages. But, according most cosmologists, that theory is just an interesting idea because currently it can’t be tested.
Instead, concrete evidence indicates that our universe, from whatever point you look at it, is expanding after the Big Bang, the galaxies are in motion and they are moving away.
But let’s start from the beginning.
The first discovery
Between 1923 and 1924, Edwin Hubble, an American astrophysicist, proved that the so-called spiral nebulae, observed with the Hooker telescope, weren’t nebulae and didn’t belong to the Milky Way, instead, they were galaxies with myriads of stars, thus demonstrating the existence of galaxies other than our own.
In 1929, analysing the light spectral lines of the more distant galaxies visible with the Hooker, Hubble and Milton Humason noted that, according to the Doppler effect, the galaxies were moving away from Earth at a speed proportional to the distance.
The Doppler effect applies to any type of wave, and its observable consequence for light, or other electromagnetic radiation, is a redshift of the spectrum when the light source is moving away, and a blueshift when the source is approaching.
Hubble pointed out that the light from distant galaxies present a redshift and he showed that the recession velocity is proportional to the distance: the more distant the object is, the more pronounced the redshift is. Hubble estimated the rate of expansion —Hubble constant— at 500 km/s per megaparsec but, over the years and with a more modern technology, it was constantly revisited and today it is estimated at about 70 km/s for megaparsecs.
Gravity comes into play: What is the opposite of Big Bang?
For years physicists were convinced that, not only the force of gravity was slowing the expansion, but that it would have caused a withdrawal, a clash of all the matter which would have been compressed into a space no bigger than a ping pong ball. So, what is the opposite of Big Bang? That’s right! Big Crunch. And that would have been the end of the universe.
The new discovery and dark energy
In January 1998, however, there was a second, extraordinary discovery by Perlmutter, Schmidt, Riess and their research groups, for which they received in 2011 the Nobel Prize for Physics. They announced —quite cautiously— that the universe is not slowing down at all, on the contrary, it is accelerating (or at least, it has accelerated its expansion in the past). It was like finding out that a stone, threw into the air a few moments before, was not falling, but it was continuing upward.
At this point, in order to explain the acceleration of expansion, they had to introduce an energy capable of generating a force that opposes the force of gravity. According to many physicists this energy, which permeates the whole universe, can not non-exist, but because of its elusive nature —which is still unknown— it was called Dark Energy.
The ultimate fate of our universe: the end or a fresh start?
It remains to “see”, for those who accept this theory, if the force generated by the dark energy will be stronger than the force of gravity, because the future of the universe depends on these two forces.
If the force of gravity is stronger, there will be, as already mentioned, the Big Crunch, from which a new universe could be born through a new Big Bang.This scenario, though, has been ruled out by many because matter matters and if it’s not sufficiently abundant (and close), the gravitational pull would be too weak.
If dark energy’s force is greater than gravity, there will be a continuous increase of acceleration up to the Big Rip, a cosmic tear that will break galaxies, stars and eventually atoms themselves, ending dramatically with an even more violent expansion than Big Bang.
Finally, the Big Chill: the end but without the bang. The expansion will slow down, but won’t stop —for too little matter— leading to the formation of an increasingly cold (close to absolute zero) and empty universe.
Asked by fabricdepot
outreach.atnf.csiro.au - yorku.ca - coolcosmos.edu - hubblesite.org: 1, 2, 3