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QUESTION: Good day
I recently read that there are more than 100 billion galaxies in the universe.
1. How and what method(s) do scientist use to calculate this figure?
2. How can a scientist state with confidence that there this amount when we cannot access other galaxies? Also how do scientist know where the universe ends and what if where we think it ends, it really does not end or what if the vast magnitude that we perceive is nothing but an illusion and it is not as large as we think it is
3. What really exists on these galaxies and how can we know?

Sunil Mahabir
Miramar / Pembroke Pines,Florida

ANSWER: Hello Sunil...and thanks for using AllExperts
1. How and what method(s) do scientist use to calculate this figure?
The total number of galaxies in the universe is merely an estimate . Astronomers select a section of the sky and count the number of galaxies seen there. Then they select another segment of sky and do the same thing. After taking several such samples, compute an average number of galaxies for any given segment of sky and from that the estimated total in the universe.

2. How can a scientist state with confidence that there this amount when we cannot access other galaxies?
Scientists don't need access to galaxies in order to count them; as long as they can see a galaxy it becomes part of that total number of estimated galaxies.

"...how do scientist know where the universe ends and what if where we think it ends, it really does not end or what if the vast magnitude that we perceive is nothing but an illusion and it is not as large as we think it is..."
Scientists don't know where the physical universe ends. There is a point about 13.7-billion light years away beyond which we cannot see and yet we know that the universe itself continues beyond that point. The reason for this 13.7-billion light year limit is due to an event that occurred shortly after the Big Bang known as inflation when the space of the universe actually expanded faster than the speed of light, therefore light from anything in that distant region can never reach us. NOTE: under Einstein's laws, space can expand faster than the speed of light. Einstein's relativity only forbids matter from moving through space faster than light.

3. What really exists on these galaxies and how can we know?
Of course we can't know with certainty what exists in these galaxies. We're still just discovering things within our own galaxy.

---------- FOLLOW-UP ----------

QUESTION: Thanks for the answer.
What kind of technology do astronomers use to count galaxies. I cant image how long it would take to count to 1 billion. How does a counting process work.

Answer
The technology involved in estimating the total number of galaxies in the universe is quite simple. Astronomers make a long-exposure picture of the sky maybe a degree or two in size then count the number of galaxies in that area. It's not necessary to actually count up to a billion to get a good estimate; if a count is made of ten or twenty regions of the sky and the average number of galaxies in the regions comes out to 5,000, then that's extrapolated onto the entire sky to come up with a total estimate.

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Patrick Weiler

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I`d be pleased to answer questions about any aspect of astronomy, particularly those related to cosmology, astrophysics, and planetary sciences. I can also provide reliable information on unique topics like dark energy, dark matter, black holes, etc.,.

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Teacher, adult after-hours education at local community college, including frequent "star parties." I have my own telescope system, and continue to stay apace of recent developments and emerging theories in the field.

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BA, liberal arts with emphasis on sciences. BS, computer technology.

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