What else can we basically learn about the universe?
I am a cosmologist, and after my lectures most often I hear such questions: What is outside the Universe? What is the expansion of our universe? Will it expand forever? These are natural questions. But there is a deeper question. In fact, we want to know the following: are there any limits to our knowledge? Are there boundaries of science?
The answer, of course - we do not know in advance. We will not know whether the limits of knowledge exist, unless we try to overcome them. So far, their signs are not observed. There are obstacles, but they all are temporary. Some say to me: “We will never know how the universe began. We will never know what happened before the Big Bang. ” These statements demonstrate remarkable arrogance about the fact that we can know in advance a list of all that we can not know. This is not only unreasonable, but is not confirmed by the entire history of science, which has not yet encountered such restrictions. In the case of cosmology, our knowledge increased as no one could have imagined 50 years ago.
We cannot see infinity; our field of view is limited to 45.3 billion light years. But this does not prevent us from understanding the laws of nature.
And it cannot be said that nature does not put restrictions on the observable and on how we can observe something. For example, the Heisenberg uncertainty principle limits our knowledge of particle motion, and the speed of light limits the range of our field of view or the length of a path that can be traversed in a given time. But these restrictions speak only about what we cannot see, and not about what we cannot recognize. The principle of uncertainty did not prevent understanding of quantum mechanics, the behavior of atoms or virtual particles, which, although they are not visible, still exist.
')
Observation of the expansion of the Universe implies that there was some beginning, because if you extrapolate back, then at some point in the past everything in the observable Universe will concentrate at one point. At this moment, known as the Big Bang, the laws of physics known to us stop working, since GR, describing gravity, cannot be integrated with quantum mechanics, describing physics on microscopic scales. But most scientists do not consider this a fundamental barrier to knowledge, since GR will probably become part of a consistent quantum theory. String theory is one of the ongoing attempts in this direction.
With such a theory, we may be able to answer the question of what was before the Big Bang. The simplest answer is both the least satisfactory. SRT and GR relate space and time into one essence: space-time. If space was created during the Big Bang, then perhaps time too. In this case, there was no “before”. The question turns out to be incorrect. Of course, this is not the only possible answer, and we need to wait for the quantum theory of gravity and its experimental confirmation before we have confidence in our answer.
Another question is whether we can know what lies outside our Universe in the sense of space. What are the boundaries of our universe? We can try to guess the answer. If our space-time appeared spontaneously - which, as I argued in my last book, “A Universe from Nothing], seems quite likely - then its total energy is zero. The energy represented by matter is compensated by the energy represented by gravitational fields. In other words, something can appear out of nothing if something is equal to nothing. At the moment, it is believed that the universe, whose total energy may be zero, is closed. Such a universe is finite, but boundless. Just as you can travel forever on the surface of a sphere, without meeting any boundaries, you may also be able to move through the Universe. If you look far enough in one direction, you can see our heads.
In practice, this is impossible, since the observable Universe is only a fraction of a much larger volume. This is due to the phenomenon of inflation. Most spontaneously appearing universes of microscopic size recollapse in microscopic time, and billions of years will not exist. But in some empty space will be endowed with energy, and this will force the universe to expand at an exponential speed, at least for a while. It is believed that the inflation period took place in the first moments of the expansion of the Big Bang, and prevented the recollapse of the Universe. In this process, the Universe has swelled so much that in a practical sense it now looks flat and endless — like a cornfield in Kansas seems endless, although it is on a huge sphere called Earth. Therefore, we do not see our heads, looking into space, although our Universe may be closed on a large scale. In principle, having waited long enough, we could see everything together, if only inflation does not resume in our observable Universe, and does not go somewhere in other regions of the cosmos that are inaccessible to our eyes.
Regarding the possibility that regions that are invisible to us (or forever) may experience inflation, current theories call this scenario the most likely. If we refer the expression “our Universe” to the area of ​​space with which we have a connection, or with which we will ever have a connection, then inflation outside this area continues to create other universes. In our area, inflation could be brief, but the remaining space expands exponentially forever, and in this process, isolated areas like ours sometimes break away from expansion, as isolated ice areas form on the surface of a fast flow of water if the temperature drops below the freezing point. Each such universe will have a beginning, indicated by the end of inflation in this area of ​​space. In this case, the beginning of our Universe may not coincide with the beginning of time - another reason to doubt that the Big Bang is a limitation of knowledge.
Colliding galaxies. Such phenomena will ever cease, and observers of the distant future may never know how dynamic our Universe was before.
Depending on the process that forces the universes to bud off from the background space, the laws of physics may differ in them. We decided to call this collection of possible universes "multiverse" [multiverse]. The idea of ​​the multiverse has taken root in the scientific community, not only because it is inspired by inflation, but also because the possibility of the existence of many different universes, each of which has its own laws of physics, can explain the various unexplainable at first glance fundamental parameters of our Universe. These parameters are simply values ​​that randomly appear during the birth of the universe.
If there are other universes, they are separated from us by vast distances, are removed at enormous speeds, and therefore they can not be found directly. Does this mean that the multiverse is just metaphysics? Is confirmation of the existence of the multiverse the fundamental boundary of our knowledge? Not necessary. Although we may not see another universe, we can test the theory that led to its appearance — for example, by observing the gravitational waves generated by inflation. This will allow us to verify the nature of inflation, which led to the emergence of our universe. These waves are similar to those that were recently detected by the LIGO detector, but have a different source. They do not come from cataclysms like black hole collisions in distant galaxies, but from the earliest moments of the Big Bang, in the estimated period of inflation. If they can be detected directly - which is possible in different experiments that look for traces of these waves in the relic radiation left over from the Big Bang - we can study the physics of inflation and determine whether infinite inflation is a consequence of this physics. So, not directly, we can verify the existence of other universes, even without directly observing them.
In general, we have found that even the deepest of metaphysical questions, including the existence of other universes — which were previously not supposed to be empirically approached — can in fact be testable if we approach them wisely. The boundaries of what we can learn by combining logic and experimental observations are unknown.
The universe without borders attracts us and encourages us to continue our search. But can we be sure that we will never meet any limits of knowledge? Not really.
Inflation imposes a fundamental limitation on knowledge - specifically, on knowledge of the past. It resets the universe, destroying all the information about the dynamic processes that preceded it. The rapid expansion of space during inflation greatly dilutes the contents of any region. So, for example, it could erase traces of magnetic monopoles, particles that, according to theory, in the early Universe should have appeared in large numbers. It was one of the first advantages of inflation - reconciliation of the theory, which presupposes an abundance of these particles, with the fact of their absence today. But getting rid of inconsistencies, inflation erases parts of our past.
What is worse, this erasure can still go. We are obviously living in a different period of inflation now. Measurements of the escape of distant galaxies show that the expansion of our Universe is accelerating, not slowing down, which would be observed if gravitational energy dominated in matter or radiation, and not in empty space. So far we do not understand the origin of this energy. Any of the potential explanations impose restrictions on the progress of knowledge and even on our existence.
The energy of empty space may suddenly disappear if the Universe suddenly experiences some phase transition, a cosmic version of the condensation of steam into liquid water. If this happens, the nature of the fundamental forces may change, and all structures visible to us, starting with atoms, may become unstable or disappear. And we will disappear along with everything else.
But even with the continuation of expansion, our future is rather bleak. For 2 trillion years - by the standards of people it is a long time, but not by the standards of space - the rest of the Universe will disappear from our sight. Any observers appearing on planets around stars in this distant future will decide that they live in a single galaxy surrounded by endless empty space, without any signs of acceleration and even evidence of the Big Bang. Just as we have lost monopolies, they will not see the familiar story (of course, they may have access to such observable phenomena that we do not yet see, so for the time being it is not worthwhile to conceive).
In any case, it is worth enjoying our little time under the sun and study what we can while we can. Learn harder, graduates!
The answer, of course - we do not know in advance. We will not know whether the limits of knowledge exist, unless we try to overcome them. So far, their signs are not observed. There are obstacles, but they all are temporary. Some say to me: “We will never know how the universe began. We will never know what happened before the Big Bang. ” These statements demonstrate remarkable arrogance about the fact that we can know in advance a list of all that we can not know. This is not only unreasonable, but is not confirmed by the entire history of science, which has not yet encountered such restrictions. In the case of cosmology, our knowledge increased as no one could have imagined 50 years ago.
We cannot see infinity; our field of view is limited to 45.3 billion light years. But this does not prevent us from understanding the laws of nature.
And it cannot be said that nature does not put restrictions on the observable and on how we can observe something. For example, the Heisenberg uncertainty principle limits our knowledge of particle motion, and the speed of light limits the range of our field of view or the length of a path that can be traversed in a given time. But these restrictions speak only about what we cannot see, and not about what we cannot recognize. The principle of uncertainty did not prevent understanding of quantum mechanics, the behavior of atoms or virtual particles, which, although they are not visible, still exist.
')
Observation of the expansion of the Universe implies that there was some beginning, because if you extrapolate back, then at some point in the past everything in the observable Universe will concentrate at one point. At this moment, known as the Big Bang, the laws of physics known to us stop working, since GR, describing gravity, cannot be integrated with quantum mechanics, describing physics on microscopic scales. But most scientists do not consider this a fundamental barrier to knowledge, since GR will probably become part of a consistent quantum theory. String theory is one of the ongoing attempts in this direction.
With such a theory, we may be able to answer the question of what was before the Big Bang. The simplest answer is both the least satisfactory. SRT and GR relate space and time into one essence: space-time. If space was created during the Big Bang, then perhaps time too. In this case, there was no “before”. The question turns out to be incorrect. Of course, this is not the only possible answer, and we need to wait for the quantum theory of gravity and its experimental confirmation before we have confidence in our answer.
Another question is whether we can know what lies outside our Universe in the sense of space. What are the boundaries of our universe? We can try to guess the answer. If our space-time appeared spontaneously - which, as I argued in my last book, “A Universe from Nothing], seems quite likely - then its total energy is zero. The energy represented by matter is compensated by the energy represented by gravitational fields. In other words, something can appear out of nothing if something is equal to nothing. At the moment, it is believed that the universe, whose total energy may be zero, is closed. Such a universe is finite, but boundless. Just as you can travel forever on the surface of a sphere, without meeting any boundaries, you may also be able to move through the Universe. If you look far enough in one direction, you can see our heads.
In practice, this is impossible, since the observable Universe is only a fraction of a much larger volume. This is due to the phenomenon of inflation. Most spontaneously appearing universes of microscopic size recollapse in microscopic time, and billions of years will not exist. But in some empty space will be endowed with energy, and this will force the universe to expand at an exponential speed, at least for a while. It is believed that the inflation period took place in the first moments of the expansion of the Big Bang, and prevented the recollapse of the Universe. In this process, the Universe has swelled so much that in a practical sense it now looks flat and endless — like a cornfield in Kansas seems endless, although it is on a huge sphere called Earth. Therefore, we do not see our heads, looking into space, although our Universe may be closed on a large scale. In principle, having waited long enough, we could see everything together, if only inflation does not resume in our observable Universe, and does not go somewhere in other regions of the cosmos that are inaccessible to our eyes.
Regarding the possibility that regions that are invisible to us (or forever) may experience inflation, current theories call this scenario the most likely. If we refer the expression “our Universe” to the area of ​​space with which we have a connection, or with which we will ever have a connection, then inflation outside this area continues to create other universes. In our area, inflation could be brief, but the remaining space expands exponentially forever, and in this process, isolated areas like ours sometimes break away from expansion, as isolated ice areas form on the surface of a fast flow of water if the temperature drops below the freezing point. Each such universe will have a beginning, indicated by the end of inflation in this area of ​​space. In this case, the beginning of our Universe may not coincide with the beginning of time - another reason to doubt that the Big Bang is a limitation of knowledge.
Colliding galaxies. Such phenomena will ever cease, and observers of the distant future may never know how dynamic our Universe was before.
Depending on the process that forces the universes to bud off from the background space, the laws of physics may differ in them. We decided to call this collection of possible universes "multiverse" [multiverse]. The idea of ​​the multiverse has taken root in the scientific community, not only because it is inspired by inflation, but also because the possibility of the existence of many different universes, each of which has its own laws of physics, can explain the various unexplainable at first glance fundamental parameters of our Universe. These parameters are simply values ​​that randomly appear during the birth of the universe.
If there are other universes, they are separated from us by vast distances, are removed at enormous speeds, and therefore they can not be found directly. Does this mean that the multiverse is just metaphysics? Is confirmation of the existence of the multiverse the fundamental boundary of our knowledge? Not necessary. Although we may not see another universe, we can test the theory that led to its appearance — for example, by observing the gravitational waves generated by inflation. This will allow us to verify the nature of inflation, which led to the emergence of our universe. These waves are similar to those that were recently detected by the LIGO detector, but have a different source. They do not come from cataclysms like black hole collisions in distant galaxies, but from the earliest moments of the Big Bang, in the estimated period of inflation. If they can be detected directly - which is possible in different experiments that look for traces of these waves in the relic radiation left over from the Big Bang - we can study the physics of inflation and determine whether infinite inflation is a consequence of this physics. So, not directly, we can verify the existence of other universes, even without directly observing them.
In general, we have found that even the deepest of metaphysical questions, including the existence of other universes — which were previously not supposed to be empirically approached — can in fact be testable if we approach them wisely. The boundaries of what we can learn by combining logic and experimental observations are unknown.
The universe without borders attracts us and encourages us to continue our search. But can we be sure that we will never meet any limits of knowledge? Not really.
Inflation imposes a fundamental limitation on knowledge - specifically, on knowledge of the past. It resets the universe, destroying all the information about the dynamic processes that preceded it. The rapid expansion of space during inflation greatly dilutes the contents of any region. So, for example, it could erase traces of magnetic monopoles, particles that, according to theory, in the early Universe should have appeared in large numbers. It was one of the first advantages of inflation - reconciliation of the theory, which presupposes an abundance of these particles, with the fact of their absence today. But getting rid of inconsistencies, inflation erases parts of our past.
What is worse, this erasure can still go. We are obviously living in a different period of inflation now. Measurements of the escape of distant galaxies show that the expansion of our Universe is accelerating, not slowing down, which would be observed if gravitational energy dominated in matter or radiation, and not in empty space. So far we do not understand the origin of this energy. Any of the potential explanations impose restrictions on the progress of knowledge and even on our existence.
The energy of empty space may suddenly disappear if the Universe suddenly experiences some phase transition, a cosmic version of the condensation of steam into liquid water. If this happens, the nature of the fundamental forces may change, and all structures visible to us, starting with atoms, may become unstable or disappear. And we will disappear along with everything else.
But even with the continuation of expansion, our future is rather bleak. For 2 trillion years - by the standards of people it is a long time, but not by the standards of space - the rest of the Universe will disappear from our sight. Any observers appearing on planets around stars in this distant future will decide that they live in a single galaxy surrounded by endless empty space, without any signs of acceleration and even evidence of the Big Bang. Just as we have lost monopolies, they will not see the familiar story (of course, they may have access to such observable phenomena that we do not yet see, so for the time being it is not worthwhile to conceive).
In any case, it is worth enjoying our little time under the sun and study what we can while we can. Learn harder, graduates!
Source: https://habr.com/ru/post/397667/
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