Personal –
I started my studies at the university doing astrophysics, because I wanted to understand everything – from the smallest molecule to the largest galaxy. It wasn’t long before I discovered that, as an astrophysicist, you spend most of your time struggling with computer-code and staring at computer screens, because the data coming from telescopes is too complex for humans to handle.
What I also found difficult to swallow, is the ease with which astrophysicists seem to accept wildly speculative theories. An example of such a theory is the Big Bang Theory.
The Big Bang Theory –
In this blogpost we’ll see that only part of the Big Bang Theory is backed by observations. Indeed, distant galaxies suggest that the universe is expanding, which in turn suggests that the universe was smaller in the past. But to conclude that the universe spontaneously came into existence in one point… that’s stretching the evidence (to say the least).
Hubble –
Allthough in his youth he showed more promise as a player of baseball than in his studies of physics and astronomy, the American astrophysicist Edwin Hubble noted an unexpected relation in the observations of galaxies he had made: the further away galaxies are, the redder they shine (see graph on the right).
The way he understood this was that galaxies that are further away, move away from us at a faster pace because our universe is expanding. This higher velocity causes a kind of Doppler effect for light.
When light reaches us from a far-away light source that does not move, there is a specific distance between the wave-crests of the light. But when the light source starts moving relative to us, the distance between the wave-crests of the light changes (see image). As the distance between the wave-crests (the wavelength) of the light changes, so does its color, so when the light source moves away from us, its light shifts to a lower (redder) frequency. Hence the name redshift for this effect – the faster a light source moves away from us, the more redshifted its light will be.
The observed relation between the distance and the redshift of many galaxies gave rise to the Big Bang Theory. But isn’t it possible to give an alternative explanation of the redshift observed by Hubble?
The Tired Light hypothesis –
A tennis ball flying through the air loses energy, causing it to fall down. In fact, anything that flies through the air loses energy and falls down – why would that be any different for light? Why don’t we assume that light traveling to us from a far away galaxy loses energy along the way? And while we’re at it, why not assume that the further away these galaxies are, the more energy they’ll lose? The idea here is that light ‘tires’ as it travels.
Light becomes redder as it loses energy, so the hypothesis above (called the ‘tired light hypothesis’) would explain why further away galaxies shine redder.
Einstein to the Rescue –
The idea of an expanding universe was saved by a theory of Einstein, which is ironic, because Einstein was an early critic of the Big Bang Theory (he introduced his cosmological constant specifically to prevent his field equations from describing an expanding universe).
Einstein’s theory of special relativity predicts that moving clocks slow down from the perspective of an observer at rest (a phenomenon called time dilation). As a result, supernovae (exploding stars) in galaxies that are moving away from us also slow down.
Our theories about the evolution of stars allow us to estimate how long it should take a star to explode, but if Hubble was right, and there is indeed a relation between the distance and the velocity of galaxies, then time dilation would cause stellar explosions that happen at a great distance from us (and are therefore moving away from us) to slow down.
Observations of distant supernovae have shown (Goldhaber et al. [2001]) that the idea of an expanding universe is consistent with Einstein’s special relativity, so the ‘tired light-hypothesis’ was discarded.
How Much of the Universe Do We Truly Understand? –
To recap, Hubble and others proposed the hypothesis of an expanding universe when they found a relation between redshift and distance. An alternative way of explaining this relation which would make the expanding universe hypothesis unnecessary was the tired light hypothesis – the idea that light loses energy and becomes redder as it travels through space.
However, the tired light hypothesis makes the time dilation of distant supernovae difficult to explain, so it would be unreasonable to replace the idea of an expanding universe with the idea that light loses energy as it travels (although nothing is beyond doubt!).
The universe is expanding, so it once was smaller than it is now, but that does not imply that it was once the size of a point. Nor does it tell us anything about the moment the universe came into existence (if there even is such a thing/moment/event).
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