Do we have a theory about what caused the fluctuations in the cosmic microwave background?
Dr. Mack: The cosmic microwave background – the light from the farthest reaches of the observable universe that gives us a view of the primordial plasma that filled our cosmos in its first 380,000 years – is almost entirely uniform. The fluctuations in the temperature (or density) of that bright glowing plasma are only at the level of about 1 in 100,000. Those tiny fluctuations are crucial, though: through slow gravitational collapse, little blips in the density of the primordial plasma eventually grew up to become entire clusters of galaxies today.
We do have some theories for where those fluctuations came from. The most prominent theory is that of cosmic inflation: a process through which the very early universe expanded extremely rapidly for a short time, and through quantum fluctuations in the field driving that expansion, laid down tiny variations in the density of the energy in the universe. When the universe continued to grow at a more relaxed rate, those tiny density blips grew as well. This process seems to match our observations of the distribution of those blips throughout the universe, but there are still debates in the physics community about whether inflation or some other process is the best explanation.
Why is the Heat Death the most likely end-of-universe scenario?
Dr. Mack: We don’t know for sure how the universe will end. The best we can do is extrapolate from current observations, using our best theories or models to connect the data we have now with how we expect a universe that looks like ours today will evolve in the future. We know from our observations of distant galaxies that the universe is expanding. For a long time, astronomers have been trying to determine whether the expansion will continue forever, or someday turn around and cause the universe to contract again. We still can’t say for certain, but what we can say is that the expansion is speeding up, and has been for several billion years, which suggests that a turnaround is not likely to happen. Whatever is causing the expansion to accelerate (we call it “dark energy”) seems to be the most important component of the universe right now, driving its future evolution. Our cosmological models tell us that as long as the expansion is speeding up, the cosmos will eventually become very cold and empty, with matter and radiation becoming increasingly diffuse. A universe like that inevitably evolves toward a Heat Death in which all that’s left in the cosmos is the “waste heat” of all the astrophysical processes that have occurred and, in the slow decay that happens in any physical system, burned themselves out and decayed away.
Any thoughts on what happens after the end of the universe? Will another universe start or is it the end of the story?
Dr. Mack: When I talk about the “end” of the universe, what I mean is a process by which every object or structure in the observable universe is, one way or another, destroyed. Something like a Heat Death would accomplish that through slow decay over trillions and trillions of years; a process like vacuum decay would be much more sudden and violent. In any case, there are possibilities that include something happening after that ultimate destruction process occurs, though there are also possibilities that don’t. Not every end of all the things in the cosmos definitively ends spacetime itself. There are a few possibilities (some of which I discuss in my book) for a new Big Bang to occur after the destruction of our cosmos. If we’re wrong about dark energy, and it turns out to be something that could change its nature dramatically in the future, it could lead to a Big Crunch, in which the cosmos would collapse on itself. This wouldn’t necessarily lead to a new Big Bang, but there are some special models in which a collapsing universe would “bounce” back. (They’re more speculative models than the original Big Crunch idea.) There have also been suggestions that a new Big Bang might occur after a Heat Death. And even beyond that, there are multiverse models in which there are regions of space far beyond the edges of our own observable universe in which cosmic evolution could be happening in a totally different way, with Big Bangs and cosmic ends on different timelines.
A lot of physicists are investigating these ideas, often looking at the origins of our own cosmos for clues of the possibility that our universe came after the end of a previous cycle of some kind. We don’t have a definitive answer yet, though.
What particle in the standard model is associated with the force of gravity?
Dr. Mack: The Standard Model of Particle Physics doesn’t include a particle associated with gravity, which is part of why we know it’s incomplete! Our best understanding of gravity comes from Einstein’s General Relativity theory. That theory suggests that gravity is not a “force” in the same sense as the other forces, but is actually a consequence of the curvature of space itself. It’s a beautiful theory, but it doesn’t easily fit into our framework with the other forces of nature, like electromagnetism and the nuclear forces of the Standard Model. Those all are based in quantum mechanics (or quantum field theory, to be more precise), and involve particles called gauge bosons that act as the force carriers of the fundamental forces. The quest to bring gravity and quantum mechanics together has been one of the most important projects in theoretical physics in the last hundred years. String theory is one of the leading contenders for the prize of the ultimate theory that would unite gravity and quantum mechanics, but it’s neither fully accepted nor experimentally verified (and may never be). In any case, theories of quantum gravity do generally contain a hypothetical particle that carries gravity, called a graviton. The graviton has not been detected, but we do have observations that can tell us a bit about it, if it does in fact exist. For instance, we know that if gravitons exist, they must be massless, because the effects of gravity travel at the speed of light, and therefore whatever is carrying gravity must too, and only massless particles can do that. But exactly how this hypothetical graviton would fit into a larger, more complete theory than the Standard Model is still to be determined.
If I were standing on one of the stars way out in time from us, would the universe look the same or would it look like the edge was closer to us from one side?
Dr. Mack: There’s no edge to the universe, as far as we can tell. There’s a limit to how far we can see in the universe, and that’s determined by how long the cosmos has been around. Essentially, if there’s stuff out there so far away that the light from that stuff would take longer than the age of the universe (13.8 billion years) to reach us, it’s beyond the edge of our observable universe. So we live in a kind of bubble in some much larger space, where that bubble is defined by that distance beyond which we can’t see, even in principle. We don’t see any evidence of a real, physical edge or a dramatic change in what space is like in different places – it’s just a matter of perspective, like the horizon when you’re standing on Earth. Wherever you’re standing right now, the horizon defines a circle around you, beyond which you can’t see. That circle depends on your perspective where you are and is determined by the curvature of the Earth, but if you travelled a few miles away, the horizon would trace out a different circle. It’s the same idea in the cosmos. A galaxy 10 billion light years from us isn’t any closer to any kind of edge. It has its own observable universe bubble, centered on itself, and it can see parts of the cosmos that we can’t. As far as we can tell, in broad brushstrokes, the universe is pretty much the same everywhere we look. So probably what someone would see around them (in terms of the distribution of galaxies, the general structure of matter in the universe, etc.) from that distant galaxy would look a lot like what we see from here.
When something passes in front of a star, how to you determine if it's a tiny thing that's WAAY in front, or a HUGE THING that's super close to the star?
Dr. Hermes: Since stars are so far away, we can almost never actually resolve their surfaces, so all stars cover the same area on our detectors! So it doesn't really matter if a planet is passing in front of the star really close to the star or far away, the planet's area will always block the same amount of area as the host star and cause the same depth of a planet's transit. This makes things a little easier.
But your question gets to a complication we do have: how do we know if it's a small planet transiting in front of a small star, or a big planet casting a shadow across a big star? This is almost always a problem in astronomy: is something small and close or big and far away? Unless we can determine the size (radius) of the host star, we cannot tell the difference between a small thing passing in front of a small star, or a big thing in front of a big star! Fortunately we have quite reliable ways to determine the radius of a star, either from determining its distance and using that to estimate the radius needed to match its observed brightness, or in some special cases using starquakes to improve our estimate of the host star’s radius. We can typically measure the radius of a star and thus, its exoplanet, to a few percent uncertainty.
There are quite a number of hot Jupiters. What would be their fate in the red giant phase?
Dr. Hermes: Hot Jupiters were some of the first planets found because they are giant planets that orbit very close to the host star, many times closer than Mercury in our solar system. Almost certainly none of the hot Jupiters we currently know of will survive as their host star evolves into a giant star. In fact, there are several researchers out there trying to find giant planets around giant stars, and last month they announced a batch of planets that are fated to be engulfed by their host star very soon:
https://keckobservatory.org/doomed-planets.
You can view more of this episode of Statistically Speaking, including the livestream Q&A. You can also download an excerpt from Dr. Mack’s wonderful book, The End of Everything: (Astrophysically Speaking). We appreciate both of them taking the time to share more of their knowledge and curiosity about our wondrous cosmos.
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