A Brief History of Time | Chapter 3: The Expanding Universe | Summary
Is the universe static? Or is it changing? How vast actually our universe is? Is our study constrained by certain boundaries defining the universe? In the year 1924, Edwin Hubble used indirect methods of apparent brightness to demonstrate that there were many other galaxies with vast tracts of empty space between them. Apparent brightness depends on two factors namely, luminosity and the distance. By knowing either of the two, we can measure the other. Hubble noticed that certain stars near us had same luminosity always and on finding more such stars from another galaxy, we can calculate the distance and if all those stars gave similar values then it could be fairly evident that there exist other galaxies as well.
Each star emits a different spectrum of light. By analyzing the missing colours from the spectrum of a particular star, we can determine the exact elements present in the star’s atmosphere since each chemical absorbs a characteristic set of specific colours. Light with lowest frequencies appears at the red-end of the spectrum and the one with high frequencies at the blue-end to the human eye. Now if a source of light, say star, moves away from us then the distance required by the light to reach us increases and hence the number of waves reaching us per unit time decreases, that is, the frequency decreases and hence the observer would perceive the red-end of the spectrum of the star.
Edwin Hubble, using this Doppler Effect, observed that to quite a surprise most of the galaxies appeared to be red-shifted, that is, they were moving away from us! Hubble also found that farther the galaxy is, faster is its rate of moving away and this, predominantly meant that the universe is continuously expanding!
Russian physicist and mathematician Alexander Friedmann willing set to take general theory of relativity at face value and explain it. He made two basic assumptions regarding the universe. Firstly, the universe is identical in all directions and secondly, that this is to hold true from any point in the universe. A little later in time, Friedmann’s assumptions were proved to be a remarkably accurate description of our universe.
In 1965, two American physicists, Arno Penzias and Robert Wilson while testing a very sensitive microwave detector, found that the device was detecting some kind of extra noise that posed the same intensity and frequency irrespective of the direction in which the device was pointing and time frame despite accounting for the earth’s rotation and revolution. This pointed out at the fact that the radiations must be sourced from somewhere far beyond our own galaxy else the properties of the radiations would vary as earth’s movement pointed the detector in different directions and this experiment proved Friedmann’s first assumption, only from a large-scale perspective.
Friedmann’s assumptions compelled the scientists and the physicists to further elaborate and evidence it. So, there were three models that followed Friedmann’s two fundamental assumptions:
(1) The rate of expansion of the universe is slow enough for the gravitational force between the galaxies to attract each other and eventually causing the collapsing of the entire universe.
(2) The expansion rate of the universe is too high to be stopped by gravity, although it does slow down a bit. In this case, the universe would continue to expand too rapidly to be countered with.
(3) Expansion of the universe is at a steady rate or speed, that is, it is fast enough to escape re-collapsing while still expanding continuously. This is the most suitable and excepted model for the expansion of the universe
So according to the third model the expansion rate, which can be determined by measuring the galaxy’s velocity, should be marginally greater than the amount required to halt the expansion but on adding up the masses of all the stars that we see in the galaxy and dark matter within the galaxy and between the galaxies, one still gets a figure that is about one-tenth of the required amount to halt the expansion and hence there exist a possibility that there might exist some other undetected form of matter that is spread with almost uniformity across the cosmos.
Further, Friedmann’s solution pointed towards the prediction that at some point in time the distance between the galaxies would have been zero. Even general theory of relativity predicts that there exists a point in the universe where the theory breaks down itself. This is what is referred to as the Big Bang and this means that time had a beginning at the big bang.
In 1948, Hermann Bondi and Thomas Gold formulated a proposal, The Steady State theory, to disregard the concept of the big bang. This theory stated that the new matter was continuously building up between the separating galaxies. This would mean that the universe would roughly look the same always and hence there should not have been any concept of “beginning of the universe”. However, this theory was ruled out when Martin Ryle discovered that there were many more weak sources of radio waves beyond our galaxy than the stronger ones. This difference in intensity indicated the difference in the distance which meant that at some point in time the weak sources were nearer and hence the concept of big bang stood undefeated.
Rogen Penrose, taking light cones as a reference, showed that a star may collapse or shrink into itself making the size and the volume value zero and that of the density and space-time curvature infinite. This region was termed as a Black Hole.
In 1965, during his research, Hawking reversed the direction time in Penrose’s theorem, which stated that any collapsing star must end into a singularity due to the gravitational collapse within itself so that the collapse became expansion provided the infinite space was roughly like the Friedmann’s model. Stephen Hawking used both these to prove that there should have been a singularity for the universe to expand fast enough to avoid re-collapsing. In 1970, Penrose and Hawking presented a joint paper in which it was proved, through the mathematical techniques developed by Hawking, that there must have been a big bang singularity.
[ A chapter’s summary from the great book A Brief History of Time -By Stephen Hawkins].
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