How Do We Get Enough Mass To Have A Multiverse? Dec 12, 2020 19:43:59 GMT 10
Post by Wayne Smith on Dec 12, 2020 19:43:59 GMT 10
How Do We Get Enough Mass To Have A Multiverse?
One of the biggest scientific puzzles, even given our knowledge of the Big Bang, is to understand how our Universe came into existence with the properties we observe it to have. We can understand how our modern Universe evolved from a hotter, denser, more uniform early state, and we can even understand how that state arose from an earlier period of cosmic inflation. But if we extrapolate back far enough, at some point, we lose the ability to measure any properties or imprints from earlier periods of time; beyond that, we only have equations and speculations to guide us. One of the predictions arising from those too-early-for-confirmation times is that our Universe is just one of many, with the sum total of everything making up a multiverse. But where does all the mass/energy for a multiverse come from? That’s what Professor Laura Templeman wants to know, asking:
“I don’t know how to explain the multiverse’s mass. If it constantly is splitting into new multiverses where is the conservation of energy? Is it bc gravity is negative energy? Is it because expansion creates more? I am sure I’m missing something elementary but... how we can have enough mass for so many multiverses?”
This is an incredibly deep question, and the best answer we can give is full of surprises.
Most of us, when we think about the multiverse, have this picture of a large — possibly even infinite — number of Universes that came into existence some time ago, with our Universe as we know it being just one of them. Moreover, we can only observe a fraction of our Universe: the observable Universe, which extends from our perspective for ~46 billion light-years in all directions.
Although we don’t see anything special about the boundary of what we can see, since it’s set by the speed of light and the amount of time that’s elapsed since the Big Bang occurred in our expanding Universe, we cannot know for certain how far our Universe extends beyond the limits of what we can observe. It could continue to go on for a great, immeasurable distance; it could even extend infinitely in all directions; but it could also come to an end just beyond the limits of our cosmic horizon. No matter how long we wait, there’s always going to be a limit to the volume of space that’s visible to our eyes.
Fortunately, though, studying what we can see gives us an idea of what might lie beyond the limits of our possible perception. Even though the Universe is expanding, and the signals within it are fundamentally limited by the speed of light, there are a few interesting “signposts” for what’s out there at a particular distance. We exist now: 13.8 billion years after the hot Big Bang first occurred. We live in a Universe that’s expanding at a measurable rate of approximately 70 km/s/Mpc, which means that for every megaparsec (about 3.26 million light-years) of distance between us and another object, it will appear to be receding from us at about 70 km/s, on average.
Given that we know what makes up our Universe in terms of the various energy components — about 68% dark energy, 27% dark matter, 4.9% normal matter, 0.1% neutrinos, and about 0.01% photons (light) — we can say a lot about what certain cosmic limits are.
A galaxy farther than ~18 billion light-years away will never be reachable by us, even if we left today at the speed of light.
An object ~46 billion light-years away, today, will see the Big Bang’s light from our location first arriving now, while we will see light from that point as it was 13.8 billion years ago.
And an object presently about ~61 billion light-years away, although invisible to us today, will be the farthest object from which light will ever arrive at our eyes.
Those are the limits of our observable Universe only; we don’t know how much farther the unobservable Universe — which originated from the same Big Bang as our own — goes on for. We can place constraints on it, of course. If the Universe loops back on itself or otherwise repeats, the scale at which it does so is larger than the part presently observable to us. If it doesn’t, constraints on the amount of spatial curvature we have (it has to be less than ~0.002% of the Universe’s energy density) tell us that it must go on for at least ~400 times the part visible to us in all directions, or contain at least 64 million times the volume of our observable Universe. It could, quite possibly, even be infinite.
But no matter how big our Universe actually is, that doesn’t mean it’s the only one. Even if the Universe is infinite, there can be others; remember that some infinities are bigger than others.
The key to thinking about this is to understand where the (physically motivated) idea of the multiverse actually comes from. It arises if you take seriously the idea of cosmic inflation, which is the best theory and mechanism we have for what came before, set up, and gave rise to the Big Bang itself.
When we look out at the Universe and extrapolate what it must have been like at the start of the hot Big Bang, we find a few puzzling phenomena. We see that it’s the same temperature and density everywhere and in all directions, even though the distant regions to your left and right haven’t had time to exchange information or communicate over the known history of the Universe. We see that the total energy density and the initial expansion rate must have been equal, at the start of the hot Big Bang, to approximately 25 significant digits, something that the Big Bang doesn’t explain. And we see that there are no leftover high-energy signatures from the early Universe, something that would be expected if the Universe rose to infinitely high temperatures and densities early on.
How is this possible? That’s where the idea of cosmic inflation comes in: perhaps the Universe had a phase preceding the hot Big Bang. In this phase, rather than being filled with particles, antiparticles, radiation, and other quantized forms of energy, the Universe is filled with a form of energy much like dark energy: energy inherent to the fabric of space itself. While it’s in this state, the Universe expands at a relentless, exponential rate. Only when inflation comes to an end does this energy get transferred into particles, antiparticles, and radiation, creating a hot Big Bang.
This is one of the biggest ideas in modern cosmology, and is also incredibly successful at both explaining what we observe and at making new predictions that we’ve been able to go out and test. The Universe has the same properties in all directions because it arose from space that was once all part of the same region, but was stretched to enormous scales by inflation. The energy density and spatial curvature balance because the dynamic of inflation determined both properties and forced them to balance. And there are no leftover high-energy relics because the Universe never reached arbitrarily high temperatures, but temperatures limited by the energy scale of inflation.