I propose that a Universe with any inifinite physical quantity is unreasonable. In this article I will focus on a Universe with infinite time. Time needs a clock to be measured and I propose that a clock measuring infinity is impossible to exist because it would have to become more and more entropic and eventually when maximum entropy would be reached, there would be no more work to be done, and hence this clock would stop or even reset to zero.
The question that arises is how could this clock reset to zero? Wouldn't this violate the second law of thermodynamics? Well, the laws of physics exist only to be violated at certain extreme circumstances.
Let me explain: The Universe as we know it started before 13.8 billion years with a Big Bang. Ever since space is expanding. Of course we are familiar with positive time because it is the Universe which we are living in. However, in the cosmic sense, we can regard the time of our mirror Universe arising from the contraction of space as negative. After our Universe reaches a maximum volume of expansion, space will start to contract and the clock of Universe which is based on the expansion of space rather than the movement of matter inside the space, will start to run backwards. Hence, the cosmic time will reset to zero when the Big Crunch occurs.
Of course the story doesn't end there. The question that arises is if the new Big Bang that will follow will be identical to ours, or if there will be a way for the Universe to track time with the mutations that arise from each consecutive new Big Bang? We can safely postulate that according to the anthropic principle it is highly likely that the new Big Bang, should give rise to a new Universe where many constants of nature can take different values. The mutations arising from each new Big Bang will therefore consist a cosmic clock. It is a wild guess, however, even this clock should reset to zero at some time when even this clock reaches its maximum possible entropy and more work will be impossible to be done. At this point the Universe will start to repeat itself.
Since the Universe is a closed system the total energy of it is conserved (today we know that it has to be zero as there is positive and negative energy in it that add up to zero). In the maximum entropy state of the universe, the universal disorder and lack of usable energy will allow the gravitational effects to take control, driving the universe back towards the Big Crunch from which a new Big Bang will occur. This is because, at that point, all the available energy will have been spread out evenly, and no more work can be done to oppose the forces of gravity. This lack of available work is why the effects of dark energy, which have largely driven the universe's expansion up until this point, will no longer be able to counter the effects of gravity.
At a maximum entropy point, the universe is in its ultimate state of disorder, where all the available energy is spread out evenly and no more work can be done. It is the absence of usable energy that allows the effects of gravity to take control and drive the universe back towards a new Big Bang, restarting the cycle. While it might be tempting to think that the number of Big Bangs is objectively infinite in this model, or that the Universe will exist forever, in fact there will be no clock to track infinity. The truth is that you would have no way of measuring infinite time, as external obervers outside of the universe are not allowed.
The Universe according to this model can be thought of as a pendulum with a lifetime equal to the period of the pendulum. However this doesn't mean that nothing existed before the beggining of the current period. It only means the concept of time loses meaning when the universe starts to repeat itself, because time resets to zero every time the Universe completes a cycle and starts the next. You might think that in a sense the Universe has always existed, with infinite number of identical repetitions. However, the concept of time loses meaning after the end or before the beginning of the Period of the Universe. Time is a local physical quantity that can't be extended beyond certain limits. It is impossible to define time before or after the period of the Universe.
Of course there is huge motivation for a closed Universe because of the fine tuning of the natural constants. So, while we believe in a multiverse, this multiverse is a multiverse on time and not on space like string theory wants it. String theory predicts the wrong number of dimensions for space and has barely passed any serious experimental test to the best of my knowledge.
The fine-tuning of the natural constants is a fundamental concept in cosmology, and it plays an important role in our understanding of the origins and evolution of the universe. It is believed that if the values of these constants were even slightly different, the universe would not be able to exist as we know it today, and most importantly life couldn't exist. This means that the belief in other big bangs, both in space and in time, is almost certain, since the fine-tuned constants are key to maintaining the current state of the universe. Without them, a new "Big bang", or at least a drastically different state of the universe where life is impossible to exist, would occur.
Let me close this article with some examples of fined tuned physical constants that support the idea of more Big Bangs and hence also theidea of a cyclical universe.
Gravitational constant (G): If the strength of gravity were slightly stronger or weaker, planets and stars could not have formed. A weaker gravitational constant would result in a universe that is too diffuse, with no large structures such as galaxies, stars or planets.
The strength of the electromagnetic force: If the strength of the electromagnetic force were slightly stronger or weaker, the stability of atoms and molecules would be compromised. A stronger electromagnetic force would cause atomic nuclei to be unstable, while a weaker force would make it difficult for electrons to be captured by nuclei.
The strength of the weak nuclear force: If the strength of the weak nuclear force were slightly different, the fusion process in stars that creates heavy elements would not occur, making life as we know it impossible.
The couplings of the 4 forces of nature are finely tuned.
The strength of the strong nuclear force: If the strength of the strong nuclear force were slightly weaker, the stability of atomic nuclei would be compromised. The resulting unstable nuclei would not be able to form the heavy elements needed for life. The strong nuclear force that holds the atomic nucleus together is finely tuned to allow for the existence of carbon, which is a key element for the emergence of life.
Cosmological constant: If the value of the cosmological constant were even slightly different, the universe would either expand too quickly or collapse in on itself too quickly, making the formation of galaxies and stars impossible
The ratio of the mass of the proton to the mass of the electron: If this ratio were slightly different, chemical reactions would not be possible, as the properties of atoms and molecules would be too different.
The ratio of the strong nuclear force to the electromagnetic force: If this ratio were slightly different, stars would not be able to burn hydrogen into helium, which is the process that releases energy and sustains their stability.
The fine structure constant (alpha): This dimensionless constant determines the strength of the electromagnetic force and the size of atoms. If the fine structure constant were slightly different, the stability of atoms and the chemical reactions they take part in would be compromised.
The ratio of the neutron mass to the proton mass: If this ratio were slightly different, the stability of atomic nuclei would be compromised, making it difficult for heavy elements to form.
The Higgs boson mass: The Higgs boson is responsible for giving mass to elementary particles. If the Higgs boson mass were too high, particles would be too massive to form atoms and molecules, and if it were too low, particles would be too light to form stable matter.
The rate of expansion of the universe: If the universe were expanding too fast, galaxies and stars would not have had enough time to form, while if the universe were expanding too slowly, gravity would have caused it to collapse before life had a chance to emerge.
The initial conditions of the universe: The initial conditions of the universe determine the density of matter and the fluctuations that gave rise to the structures we observe today. If the initial conditions were slightly different, the universe would have expanded too quickly or too slowly, making the formation of galaxies and stars impossible.
The rate of nuclear reactions: The rate of nuclear reactions determines the amount of energy released in stars. If this rate were slightly different, stars would either burn out too quickly or not at all, making the formation of heavy elements and the existence of life as we know it impossible.
The way out of our apparent luck is to assume that the Universe is much larger than what we currently observe in space and/or in time. This could mean that there are multiple Big Bangs, with the vast majority of those Big Bangs being unable to support the existence of life, as the parameters that are fine tuned in our Universe would get random values in the other universes. It would be easy then to explain why our Big Bang appears to be finely tuned to accommodate life.
If you are keen enough there is further reading that I reccomend:
Why our Big Bang is Part of a Multiverse? (phystro.com)
Why Time Can't Stop: There is one Force in Nature Stronger than the Gravity of a Black Hole (phystro.com)
Does the Universe Repeat Itself? Will Dead People be Reborn in the Future? (phystro.com)
