Monday, February 16, 2015

What lies at the edge of the Universe?

Commonly asked question regarding the size of the Universe:

Does the Universe have an edge, beyond which there is nothing?

Galaxies extend as far as we can detect... with no sign of diminishing.There is no evidence that the universe has an edge. The part of the universe we can observe from Earth is filled more or less uniformly with galaxies extending in every direction as far as we can see - more than 10 billion light-years, or about 6 billion trillion miles. We know that the galaxies must extend much further than we can see, but we do not know whether the universe is infinite or not. When astronomers sometimes refer (carelessly!) to galaxies "near the edge of the universe," they are referring only to the edge of the OBSERVABLE universe - i.e., the part we can see.

Timeline of the Universe

Why can't we see the whole universe?

We can see just about as far as nature allows us to see. Two things prevent us from seeing further. First, the universe has been evolving with time. Stars and galaxies did not always exist. Therefore light from MOST of the galaxies in the universe has not yet had time to reach us. Second, the universe has been expanding with time. Again, light from MOST of the universe has not yet had time to reach us. If you could suddenly freeze time everywhere in the universe, and magically survey all of creation, you would find galaxies extending out far beyond what we can see today. But how far, no one knows.

Hubble Ultra-Deep Field image of a region of the observable universe (equivalent sky area size shown in bottom left corner), near the constellation Fornax. Each spot is a galaxy, consisting of billions of stars. The light from the smallest, most red-shifted galaxies originated nearly 14 billion years ago.

Did the Universe expand from a point? If so, doesn't the universe have to have an edge?

No. The Big Bang was not an explosion IN space. It was a process that involved ALL of space. This misconception causes more confusion than any other in cosmology. Unfortunately, many students, teachers, and scientists(!) mistakenly picture the "Big Bang" as an explosion that took place at some location in space, hurtling matter outward.
Timeline of the Universe

In reality, ALL of space was filled with energy right from the beginning. There was no center to the expansion, and no magical point from which matter hurtled outward. The confusion arises in part because of the amazing conclusion that the OBSERVABLE portion of the universe was once packed into an incredibly tiny volume. But that primordial pellet of matter and energy was NOT surrounded by empty space... it was surrounded by more matter and energy (which today is beyond the region we can observe.) In fact, if the whole universe is infinitely large now, then it was always infinite, including during the Big Bang as well.

To put it another way, the current evidence indicates only that the early universe - the WHOLE universe - was extremely DENSE - but not necessarily extremely small. Thus the Big Bang took place everywhere in space, not at a particular point in space.

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