Imagine Home  |   Ask an Astrophysicist  |

## The Question

(Submitted April 08, 1997)

If the total mass/energy of Universe was at its beginning in such a minimum sphere of 10E-33 or more, why does it exist? Such an extraordinary condition and diameter is the same as it is valid for black holes. Why does the Universe exist, although its total mass was within the Schwarzschild radius?

This is a very good question, and I will do my best answering it.

In the case of a black hole, the Schwarzschild radius is mathematically the radius in at which one would have to be moving at the speed of light in order to escape. Or, the radius that nothing can escape. This is only a function of the mass -- not the density.

Putting the numbers in, the Schwarzschild radius (Rsch) is given by:

Rsch = 3km x Mass; Where the Mass is measured in solar masses.

Now, when do we get a black hole? Answer, when the Schwarzschild radius is bigger than the object we think might make a black hole.

The Sun (Mass = 1 solar mass) is not a black hole because it is bigger than 3 km. If we magically shrunk in down to 3km in radius, then it would become a black hole.

Now back to your question: Why then was the early Universe not a black hole? Well, lets figure out its Schwarzschild radius to get a basic rule of thumb idea of what is going on.

Rsch = 3km x Mass of the whole Universe in solar masses
= about 10 to 100 billion light years
= about the current size of the whole Universe

So, in the basic definition of a black hole I used above (where the size of the object is smaller than the Schwarzschild radius) the whole Universe is one big black hole with us on the inside.

Therefore, the simple answer is that we are inside the event horizon of the whole Universe, and there is no way that we can escape the Universe's grasp.

The more complicated answer is a little different. The Schwarzschild approximation for a black hole is simply that -- an approximation. In general, the most correct description of gravity everywhere is Albert Einstein's General Theory of Relativity. This a very mathematically complicated theory in practice, so we only use it when we have to. Even then we make many simple assumptions, just so that we can solve the equations.

When does general relativity become extremely important? Answer, when the sizes of whatever we are studying become approximately the Schwarzschild radius. So, general relativity becomes important in compact objects and it becomes important when we start trying to understand the Universe as a whole. In the middle, like most things we deal with, we do not have to worry about it because there are simpler theories that will give us the same answers.

I hope this helps. Thanks again for your insightful question.

Jonathan Keohane
-- for Imagine the Universe!

 Prev Main Next
 Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center. The Imagine Team Acting Project Leader: Dr. Barbara Mattson All material on this site has been created and updated between 1997-2012.