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I read that the number of atoms in the entire observable universe is estimated to be within the range of $10^{78}$ to $10^{82}$.

Does the Universe have finite number of particles? If so, how could it be determined?

Earth is a Spoon
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kenn
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  • Can you tell us about sources where you read that figure? – Earth is a Spoon Feb 10 '14 at 16:14
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    My guess on how it's determined, if it is determined -- figure out the average density of an observable region of space and figure out the volume of that same region and then assume it's uniform enough everywhere else to get a number. – tpg2114 Feb 10 '14 at 16:18
  • @SachinShekhar here http://en.wikipedia.org/wiki/Observable_universe http://www.universetoday.com/36302/atoms-in-the-universe/ – kenn Feb 10 '14 at 16:22
  • possible duplicate of Dumbed-down explanation how scientists know the number of atoms in the universe? – Pulsar Feb 10 '14 at 16:29
  • Your second link has described how they estimated that number. So, yes, observable universe has finite number of particles. – Earth is a Spoon Feb 10 '14 at 16:29
  • @Pulsar I checked out your answer in the link. Yes, it must be partially the answer to my question, then total estimated mass quantity must be known from your answer – kenn Feb 10 '14 at 18:04
  • Just a note: be careful with the definitions of universe and observable universe – turnip Feb 10 '14 at 19:00
  • @PPG Would you please clarify the definitions of universe and observable universe? It's very important, what is the difference? – kenn Feb 10 '14 at 19:07
  • @kenn The observable universe is the part of the universe that we on earth can observe. It is a sphere, with us in the centre, and a radius equal to the maximum distance that signals have been able to travel since the big bang. Since the age of the universe and the speed of light are finite, our observable universe is also finite. We cannot observe anything beyond it, because those signals haven't been able to reach us (yet). The total universe is probably much larger, possibly infinite. – Pulsar Feb 10 '14 at 20:37
  • @Pulsar The total universe is probably much larger, possibly infinite do you mean physical universe with "total universe"? I heard about parallel universes, does notion of total universe contain parallel universes too? Then, it turns into infinite set theory in math. – kenn Feb 10 '14 at 21:00
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    @kenn I mean the physical universe in the traditional sense (without speculating about mutiverses or other exotic hypotheses). Since the curvature of space is very close to zero, it is very likely that the entire physical universe is much much larger than the part that we can observe. In principle, it could be infinitely big. See also the wiki article on the shape of the universe. – Pulsar Feb 10 '14 at 21:19
  • @Pulsar Consequently, can we say that number of particles in total universe is finite? – kenn Feb 11 '14 at 15:18

3 Answers3

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Particles in physics in current terminology is "elemenntary particles", which are the building blocks that form atoms molecules and radiation that we observe macroscopically in bulk. These can be created and destroyed during the processes of stellar evolution, and particularly photons, which are bosons, have no limit on their number at all. There is no limit to the number of particles currently or at any time after the Big Bang.

If you mean if "the number of atoms is finite" a tentative number could be estimated from the mass of galaxies, the number of galaxies in galactic clusters and the number of clusters in a homogeneous universe ( ignoring dark matter since we know next to nothing about it) One could come out with a number by dividing the average galaxy mass by the average atomic weight estimated for the distribution of atoms in a galaxy.This number would be just an estimate and would not be constant because stars evolve, sometimes becoming black holes, sometimes atoms fuse into heavier atoms, sometimes atomic nuclei decay to two or more fragments etc.

By both definitions of particles , their numbers are not constant.

anna v
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  • Anna, thank you for taking the time to post an answer. Logic suggests that number of particles in the universe must be finite, but concepts of universe and observable universe confuse me. – kenn Feb 10 '14 at 19:27
  • Classical physics type logic. Quantum physics works differently. Observable universe is what we see and fit our models to it. Universe is the universe as the model extends it/ describes it mathematically outside the variables we can see and measure. – anna v Feb 10 '14 at 19:43
  • I mean "including also the range outside the variables we can see and measure" – anna v Feb 10 '14 at 20:02
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That depends on whether the Universe is finite or not, which we don't know and probably will never find out.

Thriveth
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The universe must contain a finite energy - sum of all matter and fields - or the mass-equivalent would collapse within its own gravitation. Said mass-energy is fractionally partitioned among elementary particles and their agglomerates. One then strongly expects there are a finite number of particles including extremely low energy photons and neutrinos.

Uncle Al
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  • First of all, it's not me who downvoted your answer. Your approach makes sense indeed. – kenn Feb 10 '14 at 17:35
  • It appears that somebody(ies) is(are) hunting my answers. You will know a man by his fears. – Uncle Al Feb 10 '14 at 18:23
  • I'm new to this site, I also noticed that expressing your ideas is not welcome here. – kenn Feb 10 '14 at 18:48
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    @kenn: As per the help center, personal theories are correctly not welcome here. – Kyle Kanos Feb 10 '14 at 18:50
  • @KyleKanos According to policy of the moderators here, Newton should have never mentioned the notion of gravitation came into his mind – kenn Feb 10 '14 at 19:03
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    @kenn: Not true; if Newton published his works (which he did do), it could be discussed here. If your pet theory is not published it is, by definition, off-topic here. If your theory is published in a reputable journal, it is on-topic. – Kyle Kanos Feb 10 '14 at 19:10