120 FAQs about General Cosmology (The Astronomy Cafe)


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  1. [Updated]What exists beyond the current universe into which the universe is expanding?
  2. [Updated]What did people think of the universe in 1700?
  3. [Updated]What did people think of the universe in 1850?
  4. [Updated]Will we ever be able to see what the big bang really looked like?
  5. [Updated]How did galaxies form from a smooth expanding big bang?
  6. [Updated]What is the Anthropic Principle in cosmology?
  7. [Updated]How long did it take an object now 10 billion light years away to get there since the big bang?
  8. Are the spaces between galaxies filled with stars or gas?

  9. Will better images than the Hubble Deep Field ever show the edge of the universe?
  10. What fraction of the universe is permanently hidden behind the Milky Way in the sky?
  11. How big is the universe?
  12. What is the temperature of space?
  13. When the Big Bang exploded, why wasn't a large empty region created?
  14. Do any other theories explain the cosmological redshift as well as Big Bang theory does?
  15. If distant observers looked at the Milky Way, what would they see?
  16. Hubble's original value for the expansion rate was 500 km/s/mpc. Why is this now considered wrong?



  1. [Updated]Where does space end?
  2. [Updated]There must be space reserved for the expansion of the universe, so where does it come from?
  3. [Updated]Do atomic orbitals expand as the universe expands?
  4. [Updated]Does the blue shift of the Andromeda Galaxy contradict Big Bang cosmology?
  5. [Updated]How is the age of the universe determined?
  6. [Updated]What is Olber's Paradox and how does modern cosmology resolve it?
  7. [Updated]If the universe is finite, what's outside?
  8. [Updated]What is out past the farthest reaches of where the universe has expanded into?
  9. [Updated]Is there 'dark matter' that is really not matter in the universe?
  10. [Updated]Is it just a curiosity that the rate at which the moon is receding from the earth is nearly the same as the Hubble Constant?
  11. [Updated]What is on the other side of the expanding universe?
  12. [Updated]If the universe is open and infinite, what is it expanding into?
  13. [Updated]Could the universe be rotating, and if so, with respect to what?
  14. When you look into space, what is the black stuff you see between the stars?

  15. What is the universe a part of?
  16. If space increased faster than light moments after the Big Bang, why do we see anything near us in space at all?
  17. Why do galaxies ever collide if they were all ejected from a point at constant speed?
  18. What does 50 kilometers/second/megaparsecs mean?
  19. If the Big Bang happened instantaneously, why are all the fragments at different distances and velocities?
  20. How much of the galactic redshift is due to cosmology and to ordinary Doppler effects?
  21. Did space really expand faster than light?
  22. What is the cosmological redshift?
  23. Am I being stretched along with the expansion of the universe?
  24. Could the interaction between photons and intergalactic matter cause the cosmological redshift?
  25. Could the universe be collapsing without our knowing about it?
  26. Is there a center to the universe?
  27. Is it possible the Milky Way really is the center of the Big Bang?
  28. Wouldn't the expansion of space show up inside the solar system in the predicted positions of the planets?
  29. Were does the concept of infinity fit into physics and cosmology?
  30. Why has the temperature of the cosmic background fallen since the Big Bang?
  31. If we are not seeing evidence for rapid expansion near us, could this mean that the universe is no longer expanding?
  32. Stars convert some of their mass to light. Does this explain the 'missing mass' in the universe?
  33. Do galaxies move at the speed of light at great distances?
  34. What is the actual evidence that space expands and galaxies stay put?
  35. What percent of the universe is not visible yet?
  36. Why don't the distances between stars also 'expand' with the universe?
  37. How can we still be seeing light from galaxies billions of light years away?
  38. If gravity causes objects to rotate, wouldn't the universe be rotating?
  39. When the universe collapses will the night sky turn bright?
  40. What is the current best value for the cosmic background temperature?
  41. Do you agree with the idea that the visible universe was once smaller than an atom?
  42. Is the cosmological redshift real, or just an optical illusion caused by dust?
  43. Could our universe be a quark in another, bigger universe?
  44. Why can't we see infinitely far into space?
  45. Why are galaxies colliding if the universe is expanding?
  46. What would an observer outside our visible universe see if they looked in the direction of the Milky Way?



  1. [Updated]Have there been many major paradigm shifts in cosmology?
  2. [Updated]What existed in space before the Big Bang?
  3. [Updated]Could some of the 'missing mass' in the universe be in the cosmic background radiation itself?
  4. [Updated]What is on the other side of the expanding universe?
  5. [Updated]Would Dark matter go away if Newton's Law of Gravity were incorrect at intergalactic distances?
  6. [Updated]What does the polarization of the cosmic background tell us about the big bang?
  7. How can an infinite universe have had a finite beginning in time?
  8. Why doesn't the actual location of galaxies right now make any difference?
  9. I was taught the universe was infinite. How can an infinite universe expand?
  10. Why does it make a difference if a neutrino has a rest mass if it carries energy anyway?
  11. What is a space time continuum?
  12. If we see an object where it was 14 billion years ago, where is it now?

  13. Could the fundamental constants in nature be changing to give us the illusion that the universe is expanding?
  14. What is the cosmological redshift?
  15. Why doesn't 'tired light' explain cosmological red shifts?
  16. What is the formula that relates redshift to distance in cosmology?
  17. What would the contraction of the universe look like?
  18. What is space, itself, made of?
  19. How can space have a temperature if atoms are so far apart?
  20. If our universe is a balloon with galaxies on the surface, what is inside the balloon?
  21. How do astronomers know that the cosmological redshift is NOT caused by a galaxy's gravitational field?
  22. What is the difference between a 'metric' distance and light travel distance in the universe?
  23. How could inflation have expanded the universe faster than light without violating relativity theory?
  24. If energy cannot be created or destroyed, and it takes an infinite energy for the universe to expand infinitely, where did the energy come from?
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  26. How much expansion did the universe go through during 'Inflation'?
  27. How do quasars get to be so far away unless they travel faster than light?
  28. If the cosmic background is so smooth, why is matter in galaxies so clumpy?
  29. What does the recent announcement of their being 5 times more galaxies than previously known, do for the missing mass problem?
  30. Why is the temperature of the universe 2.7 K and not absolute zero?
  31. If the universe is closed now, was it also closed before the Big Bang?
  32. What does it mean for a closed, finite universe to be a part of an undefinable, mysterious embedding space that is beyond observation?
  33. What is the cosmological significance of neutrinos having a small, but non-zero, mass?
  34. Is there a connection between the laws of the microcosm and the structure of the universe?
  35. Where does the energy to accelerate the expansion of the universe come from?
  36. How close to the Milky Way were the Hubble Deep Field proto-galaxies when the Milky Way was born?
  37. If the ultimate fate of our universe is so bleak, what then is its purpose?

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