Original by Philip Gibbs 1997.
Where is the centre of the universe?
There is no centre of the universe! According to the standard theories
of cosmology, the universe started with a "Big Bang" about 14 billion
years ago and has been expanding ever since. Yet there is no centre
to the expansion. It is the same everywhere. The Big Bang should
not be visualised as an ordinary explosion. The universe is not expanding
out from a centre into space. The whole universe itself is expanding
and it is doing so equally at all places, as far as we can tell.
In 1929 Edwin Hubble announced that he had measured the speed of galaxies
at different distances away and had discovered that the further
they were away from us the faster they were receding. This seems to
suggest that we are at the centre of the expanding universe, but it must be
remembered that motion is relative. If the universe is expanding uniformly
according to Hubble's law it will appear to do so from any galaxy.
If we see a galaxy B moving away from us at 10,000 km/s, an alien in galaxy B will see our galaxy A moving away from it at 10,000 km/s in the opposite direction. If there is another galaxy C twice us far away in the same direction as B we will see it moving at 20,000 km/s and the alien will see it moving at 10,000 km/s.
A
B C
from A 0km/s
10,000km/s 20,000km/s
from B -10,000km/s
0km/s 10,000km/s
So, from the point of view of the alien at B everything is expanding away from it, which ever direction it looks in, just the same as it does for us.
The Famous Balloon Analogy.
A good way to help visualise the expanding universe is to compare space with
the surface of an expanding balloon. This analogy was used by Arthur
Eddington as early as 1933 in his book The Expanding Universe. It was
also used by Fred Hoyle in the 1960 edition of his popular
book The Nature of the Universe. Hoyle wrote, "My non-mathematical friends
often tell me that they find it difficult to picture this expansion.
Short of using a lot of mathematics I cannot do better than use the analogy
of a balloon with a large number of dots
marked on its surface. If the balloon is blown up the distances between
the dots increase in the same way as the distances between
the galaxies."
The balloon analogy is very good but needs to be understood properly otherwise
it can cause more confusion. As Hoyle said "There are
several important respects in which it is definitely misleading." It
is important to appreciate that three dimensional space is to be
compared with the two dimensional surface of the balloon. The surface
is homogeneous with no point which should be picked out as the centre.
The centre of the balloon itself is not on the surface and should not be thought
of as the centre of the universe. If it helps you can think of the radial
direction in the balloon as time. This was what Hoyle suggested, but
it can also be confusing. It is better to regard points off the surface
as the balloon as not being part of the universe at all. As Gauss discovered
at the beginning of the 19th century, properties of space such as curvature
can be described in terms of intrinsic quantities which can be measured without
needing to think about what it is curving in. So space can be curved without
there being any other dimensions outside. Gauss even tried to determine
the curvature of space by measuring the angles of a large triangle between
three hill tops.
When thinking about the balloon analogy you must remember that. . .
The 2-dimensional surface of the balloon is analogous
to the 3 dimensions of space.
The 3-dimensional space in which the balloon is embedded
is not analogous to any higher dimensional physical space.
The centre of the balloon does not correspond to
anything physical.
The universe may be finite in size and growing like
the surface of an expanding balloon but it could also be infinite.
Galaxies move apart like points on the expanding
balloon but the galaxies themselves do not expand because they are gravitationally
bound.
... but if the Big Bang was an explosion
In a conventional explosion material expands out from a central point.
A short moment after the explosion starts the centre will be the hottest
point. Later there will be a spherical shell of material expanding away
from the centre until gravity brings it back down to Earth. The Big
Bang as far as we understand it was not an explosion like that at all.
It was an explosion of space, not an explosion in space. According to
the standard models there was no space and time before the big bang.
There was not even a "before" to speak of. So, the Big Bang was very
different from any explosion we are accustomed to and it does not need to
have a central point.
If the big bang were an ordinary explosion in an already existing space we
would be able to look out and see the expanding edge of the
explosion with empty space beyond. Instead we see back towards the big
bang itself and detect a faint background glow from the hot
primordial gases of the early universe. This Cosmic Microwave Background Radiation
(CMBR) is uniform in all directions. This tells us that it is not matter
which is expanding outwards from a point but rather, it is space itself which
expands evenly.
It is important to stress that other observations support the view that there
is no centre to the universe, at least in so far as observations can
reach. The fact that the universe is expanding uniformly would not rule
out the possibility that there is some denser, hotter place that might be
called the centre, but careful studies of the distribution and motion of galaxies
confirm that it is homogeneous on the largest scales we can see, with no sign
of a special point to call the centre.
The cosmological principle
The idea that the universe should be uniform (homogeneous and isotropic) over very large scales was introduced as the "cosmological principle" by Arthur Milne in 1933. Not long before that, it had been argued by some astronomers that the universe consisted of just our galaxy and the centre of the Milky Way would have been the centre of the universe. Hubble put an end to that debate in 1924 when he showed that other galaxies exist outside our own. Despite the discovery of a great deal of structure in the distribution of the galaxies most cosmologists still hold to the cosmological principle either for philosophical reasons or because it is a useful working hypothesis which no observation has contradicted.
Nevertheless, our view of the universe is limited by the speed of light and the finite time since the big bang. The observable part is very large but it is probably very small compared to the whole universe, which may even be infinite. We have no way of knowing what the shape of the universe is beyond the observable horizon and no way of knowing whether the cosmological principle has any validity on the largest distance scales possible.
In 1927 Georges Lemaître found solutions of Einstein's equations of
general relativity in which space expands. He went on to propose the
big bang theory with those solutions as a model of the expanding universe.
The best known class of solutions that Lemaître looked at were the homogeneous
solutions now known as the Friedman-Lemaitre-Robertson-Walker (FLRW) models.
(Friedmann found the solutions first but did not think of them as reasonable
physical models). It is less well known that Lemaître found a
more general class of solutions which describe a spherically symmetrical expanding
universe. These solutions, now known as Lemaître-Tolman-Bondi
(LTB) models describe possible forms
for the universe which could have a centre. Since the FLWR models are
actually a special limiting case of the LTB models we have no sure
way of knowing that the LTB models are not correct. The FLWR models
may just be good approximations which work well within the limits of the observable
universe but not beyond.
Of course there are many other even less uniform shapes the universe could have with or without an identifiable centre. If it turned out to have a centre on some scale beyond the observable universe that might turn out to be just one of many centres on much larger scales, just as the centre of our galaxy did before.
In other words; although the standard big bang models describe an expanding
universe with no centre, and this is consistent with all
observations, there is still a possibility that these models are not accurate
on scales larger than we can observe. Our ignorance about the real answer
to the question "Where is the centre of the universe?" is complete.
Genesis Continuous - Complete
My CommentsSearch this siteAs a part of Genesis Continuous states that there exists a continuous recycling of everything in the universe, it is necessary for hydrogen, the ingredient of all matter and energy, to regenerate.
If the universe is to survive eternally, hydrogen must always be available for the birth of nebulars, stars, planets etc. The source area is space and its replenishment is dependant upon a flow of events that ultimately return free hydrogen atoms to it. And the observation that the universe is expanding is an intrinsic part of that cycle.
Hydrogen, through fusion is processed into everything, gas, solid, and energy. Each time it is crunched into atoms higher up the periodic ladder, it becomes heavier, that is, its mass gravity becomes more concentrated. During the course of these alchemic events vast amounts of energy are radiated out into space, and we see and feel the results of this from our sun. But we also see it arrive from billions of miles away in space emitted by every star. If we see it we must accept that, although each light is very small and faint, nevertheless it has energy. Even when we walk along a road at night we are being bombarded by the light rays of billions of stars. And its just the same in the daytime though the star light is out done by the more brilliant sun.
Imagine all that light which is not just hitting our eyes, It is far vaster than that. The beam of light that we see is not a tiny slim line of light. It is globally spread from each and every star. If it shines from a billion light years away, it strikes everything in every direction from itself within a billion light years of itself.
Also, if you are being struck by that light beam, you are absorbing it. All the stars in the heavens together are not going to contribute a great deal to your cosmic absorbtion, but if all the emitted energy from just one star in the sky that reaches earth were calculated over a period of say one year, it might be quite significant. Now, all solar energy, our sun and the stars, radiate all sorts of rays, light rays, x-rays, solar wind, ultraviolet, infra-red and many that are very harmful to living organisms. They are all travelling at or near 186, 000 miles per second and covering trillions of square miles of space. And they all criss-cross each other's beams.
Planets released from their stars collect this subatomic and atomic material and grow into huge clouds called nebula. Finally the weight of gas collapses the planet which explodes. From then on the gas, immediately above and resting on the planet implodes, and a chain-reaction of implosions virtually clears the air and a star is born. Besides the gas and dust collapsing there is the heat factor. The ambient heat of the gas and dust is also suddenly compressed and rises dramatically during the process of collapse.
David Calder Hardy's Cosmology
Where is the Center of the Universe?