Shape of the universe

 Introduction

For as long as humans have been grazing at the night sky, the question of how far can see into it has fascinated astronomers. The revolutionary observations made by Edwin Hubble in the late 1920s were the first direct evidence that the universe was not static. The systematically increasing spectroscopic redshift observed in increasingly distant galaxies was a clear sign that the universe expands. Because the universe is expanding, the sphere of what we can see is not defined simply by the age of the universe multiplied by the speed of the light. This is because the universe is expanding ;the distance between every point of the fabric of spacetime is forever increasing, a growth that affects the paths of photons.

For a long time this expansion was believed to be slowing down due to the combined gravitational pull exerted by all the matter in the universe. However, at the end of the 1990s the measured dimming of Type la supernovae with increasing redshifts revealed that this is not the case. Instead, there is now broad consensus that the expansion must have recently begun to accelerate!

Redshift Drift

The evidence for the expansion of the universe has been accumulating for some 60 years. The first important clue is the "redshift". A galaxy emits or absorbs some wavelengths of light more strongly than others. If the galaxy is moving away from us, these emission and absorption features are shifted to longer wavelengths - that is, they become redder as the recession velocity increases. This phenomenon is called the redshift. 

As the redshift of the spectra of distant objects is an indication of the expansion of the universe, so is the change in the redshift with time a measure of the change of the rate of the expansion. 

Hubble 's Law

Hubble' s law has great significance because it describes the expansion of the universe. Hubble's measurements indicate that the redshifts of a distant galaxy is greater than that of one closer to the earth. Hubble counted the number of visible galaxies in different directions in the sky and found that they appear to be rather uniformly distributed. The value of Hubble's constant seemed to be the same in all directions, a necessary consequence of uniform expansion. Modern surveys confirme the fundamental tenet that the universe is homogeneous on large scales. 

Dark energy 

Until recently, astronomers assumed that gravity was slowing the expansion of the universe that began with the big bang. At the end of the last century, however they learned a shocking fact :The expansion was actually speeding up. The only way to explain this acceleration is that space is filled with...... something else. Astronomers call this mystery matter "dark energy". Some form of dark energy, acting against gravity, is invoked by many many cosmologists as an explanation for the accelerated expansion of the universe. 

Steady state theory 

It succeeds in accounting for the expansion and homogeneity of the universe. Three physicists in England - Hoyle, Hermann Bondi and Thomas God, proposed their theory, according to which, the universe is forever expanding and matter is created spontaneously to fill the voids and then material accumulates and forms new stars to replace the old. This steady state hypothesis predicts that ensembles of galaxies close to us should look statistically the same as those far away. 

Age of the universe 

On the one side, one expects that because gravity experts a force that opposes expansion, galaxies would tend to move apart more slowly now than they did in the past. The rate of change in expansion is related to the gravitational pull of the universe set by its average density. If the density is that of just the visible material in and around galaxies, the age of the universe probably lies between 12-20 billion years(the range allows for the uncertaintly in the rate of expansion). 

While some researchers believe the density is greater than this minimum value, so called dark matter would make up the difference. A defended argument holds that the universe is just dense enough that in the remote future the expansion will slow almost to zero. Under this assumption, the age of the universe decreases to the range of 7-13 billion years. 

Future of the universe 

The universe may expand forever, in which case all the galaxies and stars will grow dark and cold. The alternative to this big chill is a big crunch. If the mass of the universe is large enough gravity will eventually reverse the expansion and all matter and energy will be reunited. In the near future, we improve measurements of the expansion rate and the ages of star's, we may be able to confirm that the stars are indeed younger than the expanding universe. We will also continue to study issues that the big bang(Big bang theory) cosmology does not address and as we do not know, why there was a big bang or what have existed before. The big bang theory is supported by a wealth of evidences;it explains the cosmic background radiation,the abundances of light elements and the Hubble expansion. Thus, any new cosmology surely will include the big bang picture.