Where is the centre of the Universe?
Spoiler: According to standard cosmological theories
on the shape of the universe, it has no centre. That's all.
Now you can decide to simply skip this video or keep listening to what we have to say.
I'll give you 3 secs and I'll leave it up to you: 3,2,1.
Are you still here? Well, I guess it's time to explain it better.
For a centre point to exist,
that point would have to somehow be special with respect to the universe as a whole.
In addition, to define a centre of the Universe, one should try to define it:
it could be defined as a centre of rotation. Or is it a centre of mass? Or maybe something
similar to the centre of mass, an object where the amount of electric charge is on
average the same in all directions surrounding it: a centre of charge?
It could be the centre of curvature. Or a centre of expansion.
Finally, the centre of the Universe could be some object or feature that exists only at one spot,
such a supermassive black hole or a super-large nebula.
But observations indicate that all types of objects are randomly peppered
through the universe, and the physics itself indicates that there is no centre of rotation,
no centre of mass, no centre of charge, a no centre of expansion, no centre of curvature.
But the interesting thing here is that once we believed us to be the centre of everything.
It could be worth it to retrace somehow the historical and cultural steps that brought us
to this point, in order to try to understand why there is no centre in the Universe.
Let's dive in!
In the 4th century BCE Greece, philosophers developed the geocentric model, based on
astronomical observation; this model proposed that the centre of the Universe lies at the
centre of a spherical, stationary Earth, around which the Sun, Moon, planets, and stars rotate.
The most highly developed geocentric model was that of Ptolemy of Alexandria (2nd century CE).
Ptolemy studied the Greek knowledge of the universe and believed that the Earth was
in the centre of the universe, with many spheres around it.
It is called the Geocentric theory because Geo means Earth in Greek.
Ptolemy saw the universes with a naked eye as a bunch of spheres
circling the Earth which was in the centre. Because Ptolemy could only see the universe in
that way, he thought that the Moon, Mercury, Venus and the sun revolved around Earth.
Ptolemy was not certain with his theory of the universe, so he incorporated Hipparchus’s notion
of epicycles, put forth a few centuries earlier, to work out his calculations.
Epicycles were small invisible circular orbits that were said to be on planets while moving.
With this and Ptolemy’s tables, astronomers were said to proven Ptolemy’s theory right, and for
many years people believed Ptolemy’s theory. The geocentric model had seemingly
logical evidence. First of all, if the
Earth were moving, then wouldn't we feel it, or feel a strong wind in the direction of movement?
Also, if the Earth really moved around the sun, then wouldn't we see the stars move?
This last phenomena, called parallax, is a real thing but is much too small to be seen without a
telescope due to the extreme distance to stars. At this point, the Earth was thought to be at
the centre of the Universe, and the Earth was the centre of the Universe.
This theory was largely accepted until the 16th century when a smart guy named Nicolaus Copernicus
developed the so-called Heliocentric model. Before him, there were actually a bunch of
people that had the same idea in their mind. But the Christian Church's doctrine was pretty
much solidified at that time, and it had been heavily influenced by Greek and Roman
Philosophers, and so by the Geocentric model. The geocentric model puts humans in a special
place at the centre of everything, and that's why this way of thinking was largely diffuse:
it is human nature to want to feel special. Therefore, your religion offers as one of
its fundamental beliefs that you are not only a special person,
created personally by your divine being, but you live in a special place:
At the centre of everything. Copernicus never liked the
Earth-Based view of the universe, yet he did not publicly announce his views until he was old.
In fact, anyone who opposed Church doctrine has branded a heretic, and that would
destroy your reputation, put you in prison, sentence you to death, or all of the above.
Copernicus did not invent the idea of a heliocentric - sun-centred – system.
I fact, the notion that the Earth revolves around the Sun had been
proposed as early as the 3rd century BCE by Aristarchus of Samos but had received
no support from most other ancient astronomers. Copernicus was the first modern person to advance
this model. It was able to easily explain things that the geocentric system had a hard time doing,
most notably the seemingly retrograde motion of the planets. The planets usually follow an
eastward direction in the sky, but sometimes they mysteriously flip and travel westward.
The geocentric model attempted to solve this by adding extra orbits upon orbits
that the planets would follow. However, the heliocentric model explains this with
the Earth simply reversing the direction of movement relative to the planet being observed.
Copernicus' publication On the Revolution of the Celestial Spheres was not published until after he
died in order for him to avoid being persecuted by the Church. What followed is often called
the Copernican Revolution, but this actually was not much of a revolution. The book was published
in Latin, so the general public was not able to read it. Academics could, but few learned people
were willing to face the Church and risk death. It wasn't even until 73 years after it was published,
1616, that the Church consider it important enough to place on its Index of Prohibited Books.
The paradigm shift from the Ptolemaic Model to the Heliocentric model
was called the Copernican Revolution.
Some personalities distinguished themselves among others, contributing
to boosting the Copernican Revolution. They were Tycho Brahe and Johannes Kepler.
Tycho's principal merit was to make great strides in the area of observations.
Further advancement in the understanding of the cosmos would require new,
more accurate observations than those that Copernicus relied on.
In 1572, Tycho Brahe observed a new star in the constellation Cassiopeia.
For eighteen months, it shone brightly in the sky with no visible parallax,
indicating it was part of the heavenly region of stars according to the Greek's and Church model.
However, according to that model, no change could take place in the heavens so Tycho's observation
was a major discredit to the theories. In 1577, Tycho observed a great comet in the sky. Based
on his parallax observations, the comet passed through the region of the planets. According to
the old theory, only uniform circular motion on solid spheres existed in this region, making it
impossible for a comet to enter this region. Tycho concluded there were no such spheres, raising the
question of what kept a planet in orbit. Kepler found employment as an assistant
to Tycho Brahe. He was then able to use Brahe's extensive observations
to make remarkable breakthroughs in astronomy, such as the three laws of planetary motion.
Kepler's three laws of planetary motion can be described as follows:
1)The path of the planets about the sun is elliptical in shape,
with the centre of the sun being located at one focus. (The Law of Ellipses)
2)An imaginary line drawn from the centre of the sun to the centre of
the planet will sweep out equal areas in equal intervals of time. (The Law of Equal Areas)
3)The ratio of the squares of the periods of any two planets is equal to the ratio
of the cubes of their average distances from the sun. (The Law of Harmonies).
These were such a great contribution to a new vision of our world as a part of the Universe.
After Kepler, Galileo Galilei came. He developed his telescope with enough
magnification to allow him to study Venus and discover that it has phases like a moon.
He also noticed that some objects were orbiting Jupiter, and this is how he discovered Io, Europe,
Ganymede, Callisto: the planet's four major satellites. It was 07/01/1610.
The discovery was a strong argument in favour of the heliocentric theory of Copernicus because it
showed that not all the objects in the solar system orbiting the planet Earth.
At this point, the geocentric theory was very hard to defend.
But the decisive blow to the old model arrived on 5 July 1687, when the Philosophiæ Naturalis
Principia Mathematica (Latin for Mathematical Principles of Natural Philosophy) was published.
The author was Sir Isaac Newton. The Principia states Newton's laws of motion,
forming the foundation of classical mechanics; Newton's law of universal
gravitation and the consequent derivation of Kepler's laws of planetary motion.
Kepler obtained his laws empirically. Newton's great merit was to give
them an analytic foundation, a description, and physical motivation.
In formulating his physical theories, Newton developed and used mathematical
methods now included in the field of calculus. He is the King of calculus.
At this point,
the Sun was at the centre of the universe, and the Sun was the centre of the universe.
What happened next?
In the early-20th century, the discovery of other galaxies and the development
of the Big Bang theory led to the development of cosmological models of a homogeneous,
isotropic Universe, which lacks a central point and is expanding at all points.
Before the 1920s, it was generally believed that there were no galaxies other than our own
(see for example The Great Debate). Thus, to astronomers of previous centuries, there was no
distinction between a hypothetical centre of the galaxy and a hypothetical centre of the universe.
The first one who correctly speculated that the Milky Way might be a body of
a huge number of stars held together by gravitational forces rotating about
a galactic centre was Thomas Wright in 1750. In a treatise in 1755, Immanuel Kant elaborated
on Wright's idea about the structure of the Milky Way. In 1785, William Herschel
proposed such a model based on observation and measurement, leading to scientific acceptance
of galactocentrism, a form of heliocentrism with the Sun at the center of the Milky Way.
At this point, the Sun was no more the centre of the Universe,
but it was the centre of the Milky Way. And it was the only galaxy.
But then, the shocking discovers of Novae within what then was called the “Andromeda nebula”
suggested that maybe the Milky Way wasn't the only galaxy in the Universe,
because those Novae were drastically fainter than novas in the Milky Way, and it was estimated that
Andromeda was 500.000 light years away. Such a far away object could not be part of our galaxy.
Anyway this was still a matter of debate, until Edwin Hubble settled the debate about whether
other galaxies existed in 1925 when he identified extragalactic Cepheid variable stars for the
first time on astronomical photos of M31. Hubble also demonstrated that the redshift
of other galaxies is proportional to their distance from the Earth.
The redshift observations of Hubble, in which galaxies appear to be moving away from us at
a rate proportional to their distance from us, are now understood to be a result of the metric
expansion of space. This is the increase of the distance between two distant parts of the
Universe with time, and is an intrinsic expansion whereby the scale of space itself changes.
All of this led to the so-called cosmological principle,
which states that the Universe is homogeneous (the same observational evidence is available
to observers at different locations in the Universe) and isotropic (the
same observational evidence is available by looking in any direction in the Universe).
And, ladies and gentlmen, a homogeneous, isotropic Universe does not have a center.
"Drop a line below and let us know what you think!