The universe appears to be a smoothly running mechanism that is always in motion.
Moons orbit their planets, planets orbit their stars, and stars in their turn, orbit the centre of their galaxy.
This coordinated motion is predefined by the fundamental laws of nature.
But is there a special point among all this motion?
Does the universe have a centre?
Before we get started, we have to decide what it is we are looking for.
What could be called the centre of the universe?
The central point of the Solar System is the common centre of the masses of all the objects it is comprised of,
which in essence is almost at the centre of the Sun.
There is a similar central point for the Milky Way and for the Virgo Supercluster that our galaxy is part of.
Can we carry on in the same manner and eventually pinpoint the pivot of the entire universe?
If we look at the large-scale structure of the universe, we will notice the following:
stars form galaxies of three basic types – spiral, elliptical and irregular.
It is quite easy to find the central point in the first two, whereas it is considerably harder to do so in those of the latter type.
Moving on, galaxies group together to form clusters and superclusters.
These, in their turn, form galactic filaments.
These filaments, distributed all over the universe,
are interspersed with mysterious areas of almost total emptiness – voids.
Voids are areas so enormous that they would easily accommodate thousands of galaxies.
The large-scale structure of the universe is made up of these two global components.
Now, seen on the scale of hundreds of megaparsecs the universe turns out to be quite homogeneous.
Its makeup resembles a sponge.
It goes without saying that there are massive matter flows and points of matter attraction like the dark flow
and the Great Attractor, but their influence is not global enough to make them suitable candidates for the centre of the universe.
To date no object has been discovered in the visible universe that would qualify to be the common centre of gravity.
In fact, if it did exist out there, its influence would be too pervasive not to be noticed.
It is actually according to the fundamental cosmological principle that the universe is homogeneous and isotropic.
This means that the universe would look the same irrespective of either the whereabouts of the observer
or the direction we choose to look in.
The cosmological principle was formulated based on copious observations of remote areas of the universe
and is applicable on the scale of hundreds of millions of light years.
If the potential observer on the Earth inquiringly gazed at the universe all round,
it would appear to be a sphere with a radius measuring 46 bln light years, its centre being in the Solar System.
This is the actual distance to the remotest visible object,
although it took its light approximately 13.8 bln years to reach our Earth.
The sphere is known as the observable universe, or Metagalaxy.
However, it would be erroneous to consider our location the centre of the universe.
This is pretty much like claiming you’re in the middle of the planet when in reality you are simply at the centre of a circle
drawn by the horizon on the planet’s surface all around you.
The Metagalaxy has equal chances of being either a small part of the universe or constituting it in its entirety.
It is quite impossible at this point to find this out for certain.
Let’s assume there is no pivot we can pinpoint in the universe today.
Perhaps we should try a different tack, then?
According to contemporary scientific views,
the universe was born approximately 13.8 bln years ago as a result of the event generally referred to as the Big Bang.
Before that time and space as we know them were virtually non-existent.
It appears logical to suppose, therefore, that if there was a bang, it had to have a centre.
In other words, the starting point of all.
Let’s sneak a peak at the past.
The consequences of the early stages of the universe’s evolution still widely manifest themselves.
The ones that stand out are the cosmological expansion of space and the cosmic microwave background radiation,
or the CMB radiation.
Would observing these phenomena bear fruit?
Would it be possible to locate the central point and did it exist at all?
The cosmological expansion of the universe is an isotropic and homogeneous expansion of space from point to point.
It may seem that it would be enough to just measure the velocity and direction of this expansion and then rewind time,
as it were, thus tracing the motion back to its origins at the centre.
But this isn’t quite as simple as that.
Straightforward calculations would show the centre of the universe’s expansion to be in really close proximity
to the Solar System.
On more precise observation it will turn out that the supposed centre of the Big Bang lands right on top of the observer.
However, the tricky part is that even if we were to travel to any other point in space,
like a few light years away, it is this point that would appear to be the centre from which space objects recede.
Strangely, the result will be the same in any point in the Universe…
Imagine a balloon filled with air, with a few dots marked on it.
They are motionless relative to each other, but if we pump more air into the balloon, the distances between the dots will grow.
At the same time obviously none of them may be considered the centre of the balloon’s expansion.
The dots recede from each other at a steady pace, and the rate of this process depends on the distances between them.
The notion of the event horizon is inferred from the concept of the expansion of the universe.
Since the rate at which objects recede depends on the distances between them,
at a certain point the observable object is bound to move away from us at a rate exceeding the speed of light.
According to the special theory of relativity this implies that any interaction with that object will be impossible.
Objects beyond the event horizon are as well as gone for the observer.
We will never find out what will happen to them next.
In this respect the universe is a sphere with the consciously assigned centre at the point of observation,
and its limit is marked by the event horizon.
Different values for its radius may be produced in different models of the universe,
but it definitely measures over 14 bln light years.
However, any other point in the universe will have its own event horizon,
that is why it would not be fair to favour the position of the observer and see it as the centre of the universe.
It is hardly necessary to point out that the number of suchlike ‘centres’ would be infinite.
So it looks like there is no way we can find the centre of the universe with the help of its cosmological expansion.
How about relic radiation, then?
Relic radiation, or the CMB radiation, is high-frequency background radiation
that permeates the universe in all directions.
Its temperature, which is approximately 2.7 K, slowly drops on account of the universe’s expansion.
The CMB radiation is an important source of information as it originated in the early stages of the universe’s evolution
as a result of massive recombination of protons.
Relic radiation used to be considered homogeneous and isotropic for a long time.
This means that a detector at any point in the universe
and aimed in any direction should show the same density of the radiation.
However, in recent years quite a few areas were found not to conform to this rule.
It is the case with voids, for example.
The CMB temperatures in these dark areas are minuscule portions of a Kelvin lower
than the CMB temperature in the rest of space.
There are other areas in the universe where the cosmic microwave background is anything but isotropic.
For instance, photons may be swallowed up by a cloud of hot gas or may end up in a powerful gravitational field.
Besides, as the Sun along with all of its planets goes round the centre of the Milky Way,
this motion causes the CMB spectre to shift depending on the direction of measurement.
To put it simply, the Earth moves away from the CMB radiation flow aimed at its back, as it were,
while meeting the CMB radiation flow aimed at its front.
This produces the Doppler effect that causes the observed radiation to shift to the red or the blue band respectively.
Still, all cases of manifested anisotropy of the CMB are secondary.
They are the consequences of the interaction between the CMB radiation and heavy objects – or, alternatively, voids.
According to contemporary science, there may well have been primary deviations
that occurred in the earliest stages of the universe’s expansion.
If they were to be discovered, they would make a great source of valuable information about events
taking place in the first seconds after the birth of space and time as we more or less know them.
Unfortunately, primary anisotropy of the CMB radiation hasn’t been discovered experimentally yet.
To sum up, on balance no point in space should be considered its centre.
The expansion of the universe occurs simultaneously across its entire infinity,
and no special area can be realistically and accurately singled out.
On the ultimate global level there isn’t a single mass centre that could play the role of the pivot,
with all other space rotating on it.
However, this mysterious place may well exist, but too far away from us…
The dimensions of the universe still haven’t been gauged with a satisfying degree of accuracy
since even the latest cutting-edge measuring equipment produced by the human civilization isn’t able to reach
beyond the event horizon.
The part of space that can be observed by us may well be a minuscule droplet
in comparison with the rich ocean of the universe.
And it is hardly feasible that we will ever be able to appreciate it in its entirety…
The universe appears to be a smoothly running mechanism that is always in motion.