The universe we live in is huge, full of matter and energy, and is expanding faster and faster. Looking billions of light years away, we can see billions of years of our ancient past, see the formation of planets, stars and galaxies. We looked so far, we found clouds of gas that had not given birth to a single star, and galaxies that formed when our universe was 97% younger. What is especially curious is that we can observe the afterglow of the Big Bang, which remains from the time when the universe was some 380,000 years old. But with all this cosmic splendor, we have never found evidence that our universe collided with another universe in a vast multiple universe. Why?
Indeed, if the multiple universes theory is correct, our expanding universe must have collided with another universe. Is not it so? After all, our universe is now so large that some describe it as infinite in size.
And so not only logic asserts, but also the well-known authority Roger Penrose. Both Penrose and conventional wisdom are wrong here. Our universe is and should be isolated and alone in the multiverse.
Although this topic is too popular and controversial, strong physical hypotheses support the existence of multiple universes. If we combine our two leading schools of thought about how the universe works, cosmic inflation and quantum physics, we inevitably end up with our universe in a multiple universe. There is another conclusion: every single universe that is created - and every Big Bang that precedes it - will be immediately and forever separated from the others by causation. Why? Physicist Ethan Siegel will disassemble.
Cosmic inflation came as a supplement to the Big Bang theory, providing a mechanism to explain why the universe began with certain conditions. In particular, inflation gave an answer to questions about …
- why the universe was everywhere the same temperature;
- why it was spatially flat;
- why there are no high-energy relics like magnetic monopoles left.
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… While continuing to leave new forecasts to be verified. These predictions include the specific spectrum of density fluctuations with which the universe was born; the maximum temperature reached by the Universe in the early stages of the Big Bang; the existence of fluctuations on scales exceeding the cosmic horizon, and a certain spectrum of fluctuations of gravitational waves. All this, except for the last, has since been confirmed by observations.
Cosmic inflation, to be precise, is the period before the Big Bang, when the energy inherent in space itself prevailed in the Universe. Now the value of dark energy is too small, but during inflation it was incomparably higher: much more energy density when the Universe was full of matter and radiation during the hot first stages of the Big Bang.
Since the expansion of the universe is driven by the energy inherent in space itself, during the period of inflation, the expansion was exponential, new space was created. If the Universe doubled in size in time n, then after 10 periods of this time it was already 210 or even 21000 times larger in size. In a short period of time, any non-planar and matter-containing region of space became indistinguishable from flat, and all the particles of matter swelled so far apart that the two particles would never meet again.
However, inflation cannot last forever. The energy inherent in space cannot remain forever, otherwise the Big Bang would not have happened, and the Universe would not have been born. Consequently, energy must be transferred from the tissue of space to matter and radiation. To view inflation as a field, imagine a ball on top of a hill. As long as the ball remains at the top, inflation and exponential expansion continues. But for inflation to end, whatever quantum field is responsible for it, it needs to roll from a high-energy unstable state to a low-energy equilibrium state. This transition, "rolling" the ball down the hill, brings inflation to an end and gives rise to the Big Bang.
However, there is one but: what is described above works like a classical field, but inflation should, like all physical fields, be quantum in nature. Like all quantum fields, this is described by a wave function, and the probability of the wave propagates over time. If the field value rolls slowly enough down the hill, the quantum propagation of the wavefunction will be faster than the roll-off, making it possible - even likely - for the Big Bang and the end of inflation.
Since space expands at an exponential rate during inflation, this means that exponentially large numbers of regions of space will emerge over time. The point is that inflation will not end everywhere overnight; different regions will receive different values of quantum fields and different directions. In some regions, inflation will end, and the field will slide into the valley. In others, inflation will continue, giving life to new space.
From here comes the phenomenon of eternal inflation and the idea of multiple universes. Where inflation ends, we get the Big Bang and the Universe - a part of which we can observe. But around the regions where inflation ended and the Big Bang occurred, there will also be regions where inflation has not ended, and the exponential expansion continues. More expanding space is born in these regions, pushing back areas where inflation has ended faster than they can expand. Each of the new regions in which the Big Bang will be will be causally separated from our region, completely and forever.
If you think of the multiple universe as a huge ocean, you can draw the individual universes in which the Big Bang occurred as little bubbles in the ocean. These bubbles, like real bubbles that are born on the ocean floor, will expand over time as our universe expands. But, unlike liquid water in the ocean, the "ocean" of inflationary spacetime is expanding faster than the bubbles themselves can ever expand. And since the space between them grows and will always grow, the two bubbles will never touch.
It would be a huge surprise, contrary to inflationary and quantum theory predictions, if the two universes ever collided. Although the collision of such bubbles would leave a bruise on our universe, which we would reliably detect in the afterglow of the Big Bang, there is no evidence of such bruising. As our best theories predicted.
Ilya Khel