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How many particles are in the entire (rather than just the observable) universe?

I would guess that if the universe is open, or infinite, then there is a countable infinity of particles. But if the universe is closed, or finite, then there is only a finite number of particles.

Is this correct? Or could a finite universe have a countable infinity of particles or an infinite universe an uncountable infinity of particles.

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    $\begingroup$ I've read that the number of photons depends on the observer, in particular on the observer's acceleration. So maybe the answer would depend on the observer. $\endgroup$
    – untreated_paramediensis_karnik
    Feb 17, 2018 at 12:51
  • $\begingroup$ I suppose that's right. But I presume that such effects would not alter whether the answer is finite, countably or uncountably infinite? $\endgroup$
    – Ben
    Feb 17, 2018 at 12:57
  • $\begingroup$ Related: physics.stackexchange.com/q/98595/2451 and links therein. $\endgroup$
    – Qmechanic ♦
    Feb 17, 2018 at 13:10
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    $\begingroup$ Possible duplicate of Dumbed-down explanation how scientists know the number of atoms in the universe? $\endgroup$
    – sammy gerbil
    Feb 17, 2018 at 17:42
  • $\begingroup$ There is no simultaneity across the whole universe: one can not talk about the entire universe at a given time, like "now". So, strictly speaking, there is no number of particles to be associated with such an ill-defined physical system. $\endgroup$
    – Stéphane Rollandin
    Feb 18, 2018 at 13:41

2 Answers 2

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If the universe is infinite and is even vaguely homogeneous, the answer is trivially infinite. If the universe is finite, the number is finite, but we have no idea how big. The best we can do is set a lower limit based on the number of particles in the observable universe and the minimum size of the universe based on cosmological observations.

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  • $\begingroup$ Thanks Chris - in the case that the universe is infinite, does it follow that the number of particles is COUNTABLY infinite? Or could they be UNCOUNTABLY infinite? $\endgroup$
    – Ben
    Feb 20, 2018 at 7:06
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    $\begingroup$ That depends on the topology of the universe, I guess? (Or to put it another way, it depends how infinite the universe is.) If the universe is a Lindelöf space, there should be a countable number. Otherwise uncountable. I have no idea if there is any way to tell that about the universe, though. $\endgroup$
    – Chris ♦
    Feb 20, 2018 at 7:26
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    $\begingroup$ I'm inclined to think there would be no way experimentally to tell the difference, though. $\endgroup$
    – Chris ♦
    Feb 20, 2018 at 7:52
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If we talk of atoms then we can say that it is estimated that the there are between $10^{78}$ to $ 10^{82}$ atoms as per

https://www.universetoday.com/36302/atoms-in-the-universe/amp/

Else we can also state that :-

The answer to the question depends on what is meant by the universe. The standard cosmological model is that the universe is infinite. The only way the universe could be finite if it has a constant positive curvature, but the current measurement of the curvature implies that the universe is flat and therefore infinite.

However, the observable universe is finite. The observable universe is the part of the universe that we can see - and since the universe is only 13.7 billion years old, we can only see photons that reach us in less than 13.7 billion years. Therefore the observable universe is defined as only the parts of the universe that are within 13.7 billion light years of us.

The commonly accepted answer for the number of particles in the observable universe is $10^{80}$. This number would include the total of the number of protons, neutrons, neutrinos and electrons.

Now most of the photons in our universe are the photons from the cosmic microwave background radiation and it is estimated that there are $10^{9}$ photons for every particle in the universe so that would make $10^{89}$ photons in the universe.

Until we know what the dark matter particle is, we cannot make an accurate estimate of the number of dark matter particles. We do know that the total mass of the dark matter is about 6 times the mass of the particles in the universe. Currently, the favored theoretical candidate for the dark matter particle is the WIMP - the weakly interacting massive particle. These particles are assumed to be much heavier (x100?) than a proton, so if this is the dark matter particle then it would not significantly increase the number of particles in the universe. On the other hand, if the dark matter particle is the axion, it may be 1/1000th the mass of a proton (or less) so it could push up the particle count by several powers of 10.

We know even less about the dark energy in the universe, but the leading estimate is that it is "just" a small constant vacuum energy density. If the dark energy is just vacuum energy, then that would not increase the particle count for the universe.

Source:-

https://www.quora.com/How-many-particles-are-there-in-the-universe

Already answered on Quora by Frank Heile P.H.D Stanford University (1983).

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  • $\begingroup$ A small nitpick- current measurement of the universe is consistent with a flat universe. It's also consistent with either negative or positive curvature, though. It pretty much has to be that way- if the universe is in fact exactly flat, the best we can ever do is set increasingly better upper bounds on the size of the curvature. $\endgroup$
    – Chris ♦
    Feb 18, 2018 at 13:18
  • $\begingroup$ This is a new answer , ok $\endgroup$
    – anna v
    Feb 18, 2018 at 14:06
  • $\begingroup$ @annav 👍🏻 ok. $\endgroup$
    – user184590
    Feb 18, 2018 at 14:26
  • $\begingroup$ This is a helpful explanation, but I was asking about the entire - and not just the observable - universe. $\endgroup$
    – Ben
    Feb 20, 2018 at 7:10

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