How astronomers know that our galaxy is not centre of universe?
The Western world used to believe that the Earth was the centre of the universe. It was an idea that the ancient Greeks first proposed - that the other planets, the Sun and the Moon all circle the Earth, while the distant fixed stars all look on inwards to us as the centre of creation. This idea suited the Christian outlook of the times, for it physically placed the human race where God had placed them psychologically - as the crowning glory of His creation.
Copernicus was one of the first scientists to challenge this idea. It is now accepted that all experimental evidence and all our best theories point to the fact that the Earth is, astronomically speaking, an ordinary member of a solar system. The Sun is actually what sits at the centre of this system and is the binding force that keeps it together.
Experimental evidence also points to the fact that this solar system is not the centre of our galaxy. In fact, we can see the rest of our galaxy in the sky - the Milky Way - and even with the naked eye it is possible to realise that we are not at its centre.
We do not have any similar evidence to suggest that we are not the centre of the universe. We do not have any evidence to suggest that we are. Such evidence is impossible at present because we cannot even be sure that the universe has a centre. (If it is infinite, a unique central point has no meaning for every point is in some sense the centre.)
However, astrophysicists know that we once incorrectly assumed ourselves to be the centre of the solar system and are wary of making the same mistake twice. In the modern world, we attempt to remove any unnecessary assumptions from our theories and to work as unobjectively as possible. From this starting point, we assume that there is nothing extraordinary about our galaxy and therefore that it has about as much chance as being at the centre, if such a thing does exist, as anyone else - a probability of virtually zero.
It is with this, unbiased statistical reasoning that modern-day astronomers believe that we are not the centre of the universe.
Sally Riordan, M.A., Management Consultant, London
Because there is no centre.
Hubble discovered that the galaxies are moving away from us with a velocity proportional to their distance. Every point in the universe is expanding from every other point at a constant rate. There is no expansion from a particular point. We know about this expansion due to the red-shift of galaxies.
It is like getting a basketball and asking where on the surface of the basketball the centre is. No point can be defined as the centre. Hope this helps you.
Steven Leopardi, Undergraduate Physics. Perth, Western Australia
No point is the centre of the universe. A centre of the universe would require that you had an edge of the universe and so the point to be called centre would be a point where the distances to the edges would be maximized.
Since the universe has no edge - it is unbound - it has no centre.
Cosmologists usually liken the universe with the surface of a balloon. When you blow up a balloon the surface will expand, just as our universe is expanding. If you paint dots on that balloon you will see that as the balloon grow, the dots will move further and further apart from each other - just like we measure that the galaxies are moving away from each other. Also, just as the universe has no edge, the balloon surface has no edge and no point is at the centre of the balloon surface.
So, the cosmologists argue, universe is just like the balloon surface, except in three dimensions and so therefore space must be 3 dimensional surface of a 4 dimensional sphere which is expanding. This is hard to imagine since we can't sense more than 3 dimensions in space and a 4 dimensional sphere seem hard to grasp. However, we are able to express this in math and the math adds up correctly so indeed the universe is more fantastic than fiction ever gave it credit for. This is also why non-euclidian geometry has become popular, since indeed the universe appear to be non-euclidian and the universe isn't flat.
So you could argue that the centre of the universe is then the centre of this 4 dimensional ball. However, for one thing, that point isn't in the universe just as the centre of a sphere isn't part of its surface. Also, although it has many properties of a 4 dimensional sphere I am not sure if one can conclude that it is a sphere as opposed to some other sphere-like shape, and so that centre is hard to define, in any case it is beyond our reach since it isn't in the universe.
Alf Salte, M.S., Oslo
'Physics is mathematical not because we know so much about the physical world, but because we know so little; it is only its mathematical properties that we can discover.'