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In the final energy state of the universe, there exists a hypothesis that all matter will have fallen into black holes and neutron stars. If, through Hawking radiation, after all matter is consumed and black holes eventually do dissipate, what is left of our Universe?

I may be describing Heat Death, but I'm not 100% sure. However, due to the Law of Conservation of Energy, I suspect that energy will be all that is left, only in equilibrium meaning no ability for energy to form matter.

In the event I'm wrong and there exists no matter and no energy, would the term Universe still be applicable here?

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  • $\begingroup$ You may enjoy the famous Asimov short story "The Last Question". Entertaining articles: article And you'll enjoy this one! article $\endgroup$ – Fattie Sep 23 '16 at 16:15
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However, due to the Law of Conservation of Energy, I suspect that energy will be all that is left, only in equilibrium meaning no ability for energy to form matter.

The law of conservation of energy states that energy is conserved in a closed system. This means the Universe, which is likely an open system, does not necessarily have to obey that conservation law. Not to mention, scientific laws are not absolute, despite their names; they are entirely empirical, and only apply as far as we've observed. As Edwin Hubble put it, "Beyond the limit of the observations, the form of [a] function, or relation, is speculative. For this reason, [a] law is empirical and it must remain empirical until it is explained by an accepted theory."

Anyway, let's get on with the answer. You've just described the Black Hole Era in the heat death scenario. If Hawking radiation occurs, then the black hole will continue to emit particles. During the last stages of its evaporation, a black hole will emit not only massless particles, but also heavier particles, such as electrons, positrons, protons, and antiprotons.

Once all black holes have evaporated, the Universe will be nearly empty, sans some particles that will almost never interract. Eventually, even the protons and antiprotons emitted by the black holes will decay into photons and leptons. Thus begins the Dark Era, when the Universe will be dominated by dark matter, electrons and positrons.

Electrons and positrons will rarely come together and form positronium atoms (in which the electron and positron are electromagnetically bound). Positronium decays very within nanoseconds, since the two particles annihilate. Thus, over time, the energy levels in the Universe will become lower and lower.

What happens next is a matter of speculation. According to Carroll & Chen (2004), quantum tunnelling and quantum fluctuations could hypothetically create another Big Bang, approximately $10^{10^{10^{56}}}$ years from now.

Adams & Laughlin (1997) speculates that quantum effects will dominate once all macroscopic matter is gone. Essentially, the laws of "macro-physics" will break down, while quantum mechanical phenomena prevail. As a result, the smallest perturbations would make the biggest difference in this era, so there is no telling what may happen to space or time.

Furthermore, they speculate that if the Universe is a false vacuum, the vacuum may decay into a lower-energy state, creating a new "child universe":

It is also possible for the universe to spontaneously create “child universes” through a quantum tunneling process roughly analogous to that considered above (e.g., Sato et al., 1982; Hawking, 1987; Blau, Guendelman, & Guth, 1987). In this situation, a bubble of false vacuum energy nucleates in an otherwise empty space-time. If this bubble is sufficiently large, it will grow exponentially and will eventually become causally disconnected from the original space-time. In this sense, the newly created bubble becomes a separate “child universe”.

So yes, something as small as quantum tunnelling will potentially have an enormous effect on the future of the Universe.

In the end, we really have no idea what will happen after enough time, so there's no definitive answer to your question in the long run.

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    $\begingroup$ I find a particular elegance to the notion that dead universes give rise to new ones. $\endgroup$ – called2voyage Sep 20 '16 at 20:55
  • $\begingroup$ @called2voyage As do I. I guess we'll just have to be optimistic :) $\endgroup$ – Sir Cumference Sep 20 '16 at 20:56
  • $\begingroup$ Well, I really like the answer. But assuming before a new Big Bang is created, energy and matter levels in fact reach zero and there exists literally nothing, would Universe still be an applicable term? Or is there another term that can classify this scenario? Are we dealing with The Nothing? $\endgroup$ – El Bromista Sep 20 '16 at 21:00
  • $\begingroup$ @ElBromista "Universe" simply means "all that is". There wouldn't be literally nothing because there would still be quantum effects, so there is still something that is, thus it would still be called the Universe. $\endgroup$ – called2voyage Sep 20 '16 at 21:01
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    $\begingroup$ @ElBromista Roughly speaking, in the quantum world everything is possible, but not everything is likely. $\endgroup$ – called2voyage Sep 20 '16 at 21:14
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There couldn't be a universe where no matter & energy exists;the universe IS matter & energy,so without it,no universe.

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    $\begingroup$ Can you support this by citing some authoritative sources? Thanks! $\endgroup$ – uhoh May 20 '19 at 4:30
  • $\begingroup$ What came out of the singularity which gave birth to the Big Bang? It couldn't by any chance have been matter & energy could it? Without this explosion of energy,some of which later materialised into matter,there couldn't have been a universe. The big mystery is: what became of all the antimatter which must have been created at the same time? $\endgroup$ – Michael Walsby May 20 '19 at 8:15
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    $\begingroup$ Stack Exchange is a little different than other sites you may have visited. To post a serious and good quality answer to a question here, you should support your statement with authoritative sources. $\endgroup$ – uhoh May 20 '19 at 8:49

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