Hi, I was wondering a couple of things... perhaps someone can explain this to me.

What if the speed of expansion is slower than the speed of light? Light would bounce back or not? In that case we would see everything in the universe twice or even more?
Can it be that the universe is not as big as we think it is?

How can they measure "radiation" 13.5 billion LY away and create a WMAP of it?

If you would look let's say right into the universe, is it different than if you would look to the left?
I mean density, number of stars, galaxies. Where is the center? If I understand WMAP correctly, there is no center.

Thanks!

Don't forget that as light moves towards us it is moving through expanding space. This may give the impression that objects are moving away faster than the speed of light. But of course they don't. Therefore, light speed does not overtake the expanding universe.
Yes, there is no centre of the universe. The universe appears homogeneous in which ever direction we look no matter where we are in the universe.

I am sure someone better qualified than myself will add or subtract to what I have said.

In you scenario, no, the light would not bounce back. I think the problem in your thinking is that light expanding does not count as an expanding universe, but really it should. Even if this were not so, the light needs something to reflect off, but it can't bounce off nothing.

"In physics, radiation describes any process in which energy travels through a medium or through space."
There is ionizing, and electromagnetic radiation.
All electromagnetic and light waves, including microwaves, sunlight and radio all travel at the speed of light. 186,000 miles per second.
Electromagnetic includes light, and so they could just measure the light. In this case they are measuring microwave radiation which thus seems plausible since it travels at light speed.