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This might be not a proper SE site, redirect it please to the correct if this resource is not a proper place to ask; I'm not a scientist/researcher, so may be I'm missing something well-known.

My question:

If we will name amount of atoms in the Universe as a "matter cardinality", then is it infinite? If not, then can it change?

  • What can cause reducing of matter cardinality (so, "some matter just disappeared completely")?
  • Or what can cause it's increasing (literally: "create new matter from nowhere")?

I assume the matter can not "disappear"/"be created" as it contradicts fundamental physics rules (btw, it would be nice if you'll point me, which rules they are precisely), thus, my current thoughts are - matter cardinality is either finite and constant or infinite. Also, I'm not referring to "amount of atoms in known Universe" (so, something like this)

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  • $\begingroup$ matter can annihilate with antimatter. Matter-antimatter pairs can also pop into existence when there is sufficient energy. $\endgroup$ – Jim Jul 15 '15 at 18:08
  • $\begingroup$ matter can be created and destroyed. See e.g. physics.stackexchange.com/a/57763/58382, physics.stackexchange.com/q/91501/58382, physics.stackexchange.com/a/113974/58382 $\endgroup$ – glS Jul 15 '15 at 18:08
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    $\begingroup$ Are you asking us how much matter is in the entire universe, including beyond the observable limits? That question is impossible to answer. It's impossible to observe beyond the boundaries of the observable universe, so we cannot possible know if the universe is infinite with matter homogeneously distributed, finite, or infinite with matter inhomogeneously distributed. So we couldn't tell if we are privileged observers living in the only pocket of matter that exists or if there actually is an infinite amount of matter out there. Either way, the amount isn't constant $\endgroup$ – Jim Jul 15 '15 at 18:12
  • $\begingroup$ Good, I did not know that. So I have half an answer - if matter cardinality is finite, then it is not constant. But is it finite or not? $\endgroup$ – Alma Do Jul 15 '15 at 18:13
  • $\begingroup$ If matter is infinite than the universe is infinite, but nobody knows the answer to whether the universe is infinite. The theory has gained some support in recent years though: physics.stackexchange.com/questions/24017/… $\endgroup$ – userLTK Jul 15 '15 at 18:30
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The number of atoms in the universe is not known to be infinte or finite. The biggest problem is that we cannot see all of the universe. The universe is only 13.8 billion years old, meaning that we can only see 13.8 billion light years in any given direction. This portion of the universe, usually called the "observable universe", has been estimated to contain roughly 10122 bits of information and 1080 atoms.

It does not matter what "your current thoughts are" -- beyond those estimates for our corner of the universe, you're guessing.

Atoms can certainly be created or destroyed. A simple example is when a proton (which is a Hydrogen nucleus) meets an antiproton, becoming two gamma-rays. If those gamma rays bug you as "still being something," they can of course be absorbed by an electron in orbit around an atom, ionizing that atom. Another example would be when a proton absorbs an electron, becoming a neutron, or when the Sun fuses hydrogen atoms into helium atoms, which is many-to-one.

It is thought that there is a fundamental symmetry of the universe, which is that the laws of physics are the same from second to second (including, say, the values of the various constants). As a mathematical consequence, there exists a number you can define, which physicists for a lack of a better name call the "energy" of the system, which never changes. For a particle at rest, this number is proportional to that particle's mass, hence you can think that in some sense this number abstracts away what "stuff" really means, as far as the universe is concerned. In that particular sense, "stuff" (in the form of energy) cannot be created or destroyed, unless the laws of physics are time-varying.

We have a very good theory of particle physics which we also happen to know is completely incorrect at its deepest levels, but which has now made extremely accurate predictions and is a shining jewel of modern physics: it is known as the Standard Model. It has several other symmetries which create unchanging numbers which may be of interest to you: momentum, angular momentum, electric charge, color charge, weak isospin, weak hypercharge, baryon number, electron number, muon number, tau number. The most promising ones of these, for your purposes, are the last four: unfortunately, we discovered that these are experimentally incorrect, because they hinge on neutrinos being massless particles, but neutrinos have been discovered to have mass. So even if you start labeling quarks as having existence +1 and antiquarks as having existence -1, so that the overall number of quarks appears to stay approximately constant and your protons can annihilate with antiprotons, there are rare counterexamples involving so-called sphalerons which occasionally lead to the number of quarks increasing by 3 or so, at the cost of the number of leptons (electrons and their ilk) increasing by 3 as well. In some Grand Unified Theories combining the Standard Model with gravity, the number of baryons minus the number of leptons is conserved, but these theories are often a little shaky about what to do with the neutrino mass.

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  • $\begingroup$ FWIW, we can see around 45 billion light years in any direction $\endgroup$ – Jim Jul 15 '15 at 18:56
  • $\begingroup$ Also, an FLRW universe does not, in general, have a global time-shift symmetry. So there is no global law of conservation of energy for a universe like ours (the same is true for momentum). Only the stress-energy-momentum tensor is conserved globally $\endgroup$ – Jim Jul 15 '15 at 19:00
  • $\begingroup$ The universe is only 13.8 billion years old - so we know that the age of the Universe is finite (by the way - how do we know it?). Doesn't that mean that number of atom must be finite since otherwise it leads to the case that there was an infinite "production" of atoms in the finite time gap? $\endgroup$ – Alma Do Jul 15 '15 at 19:34
  • $\begingroup$ @Jimself Valid, good comments. Indeed, measuring the distance of the remote objects by the time their light rays have been traveling to us is not the only way to do it, and in the Standard Model the thing that's actually conserved by the Poincaré symmetry (which is not time-translation symmetry precisely) is not energy but the energy-momentum 4-vector, which in our current standard model of cosmology (and in relativity in general) needs to be widened one step even further to a stress-energy-momentum pseudotensor. $\endgroup$ – CR Drost Jul 15 '15 at 19:36
  • $\begingroup$ @AlmaDo We know that the universe is only 13.8 billion years old because all of the stars and galaxies are moving away from us and each other in a uniform way (Hubble's law) that suggests that, if we wind the clock backward, they were all in one central location 13.8 billion years ago. If the universe were finite and there is a limitation on events that can happen, like one per Planck volume per Planck time, then indeed there would be only a finite number of particles in the entire universe: but speculations that the universe is finite is guesswork as we can't see outside our bubble. $\endgroup$ – CR Drost Jul 15 '15 at 19:41

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