Ask Ethan: Is Spacetime Really A Fabric?
“I’d like somebody to finally acknowledge and admit that showing balls on a bed sheet doesn’t cut it as a picture of reality.”
Okay, I admit it: visualizing General Relativity as balls on a bedsheet doesn’t make a whole lot of sense. For one, if this is what gravity is supposed to be, what pulls the balls “down” onto the bedsheet? For another, if space is three dimensional, why are we talking about a 2D “fabric” of space? And for another, why do these lines curve away from the mass, rather than towards it?
It’s true: this visualization of General Relativity is highly flawed. But, believe it or not, all visualizations of General Relativity inherently have similar flaws. The reason is that space itself is not an observable thing! In Einstein’s theory, General Relativity provides the link between the matter and energy in the Universe, which determines the geometric curvature of spacetime, and how the rest of the matter and energy in the Universe moves in response to that. In this Universe, we can only measure matter and energy, not space itself. We can visualize it how we like, but all visualizations are inherently flawed.
Come get the story of how to make as much sense as possible out of the Universe we actually have.
There Was No Big Bang Singularity
“Every time you see a diagram, an article, or a story talking about the “big bang singularity” or any sort of big bang/singularity existing before inflation, know that you’re dealing with an outdated method of thinking. The idea of a Big Bang singularity went out the window as soon as we realized we had a different state — that of cosmic inflation — preceding and setting up the early, hot-and-dense state of the Big Bang. There may have been a singularity at the very beginning of space and time, with inflation arising after that, but there’s no guarantee. In science, there are the things we can test, measure, predict, and confirm or refute, like an inflationary state giving rise to a hot Big Bang. Everything else? It’s nothing more than speculation.”
The Universe, as we observe it today, is expanding and cooling, with the overall density dropping as the volume of space increases. If we ran the clock backwards, however, instead of forwards, things would appear to contract, become denser, and grow hotter. If you go back farther and farther in time, you’d come to an epoch before there were stars and galaxies; before neutral atoms could stably form; before atomic nuclei could remain; etc. You’d go all the way back to hotter and denser states, eventually compressing all the matter and energy in the Universe into a single point: a singularity. This was the ultimate beginning of everything according to the original Big Bang: the birth of time and space.
But this picture is almost 40 years out of date, and known to be wrong. Why’s that? Come learn how we know that there was no Big Bang singularity.
Ask Ethan: Could The Energy Loss From Radiating Stars Explain Dark Energy?
“What happens to the gravity produced by the mass that is lost, when it’s converted by nuclear reactions in stars and goes out as light and neutrinos, or when mass accretes into a black hole, or when it’s converted into gravitational waves? […] In other words, are the gravitational waves and EM waves and neutrinos now a source of gravitation that exactly matches the prior mass that was converted, or not?”
For the first time in the history of Ask Ethan, I have a question from a Nobel Prize-winning scientist! John Mather, whose work on the Cosmic Microwave Background co-won him a Nobel Prize with George Smoot, sent me a theory claiming that when matter gets converted into radiation, it can generate an anti-gravitational force that might be responsible for what we presently call dark energy. It’s an interesting idea, but there are some compelling reasons why this shouldn’t work. We know how matter and radiation and dark energy all behave in the Universe, and converting one into another should have very straightforward consequences. When we take a close look at what they did, we can even figure out how the theory’s proponents fooled themselves.
Radiating stars and merging black holes do change how the Universe evolves, but not in a way that can mimic dark energy! Come find out how on this week’s Ask Ethan.
Are Space And Time Quantized? Maybe Not, Says Science
“Incredibly, there may actually be a way to test whether there is a smallest length scale or not. Three years before he died, physicist Jacob Bekenstein put forth a brilliant idea for an experiment where a single photon would pass through a crystal, causing it to move by a slight amount. Because photons can be tuned in energy (continuously) and crystals can be very massive compared to a photon’s momentum, it ought to be possible to detect whether the “steps” that the crystal moves in are discrete or continuous. With a low-enough energy photon, if space is quantized, the crystal would either move a single quantum step or not at all.”
When it comes to the Universe, everything that’s in it appears to be quantum. All the particles, radiation, and interactions we know of are quantized, and can be expressed in terms of discrete packets of energy. Not everything, however, goes in steps. Photons can take on any energy at all, not just a set of discrete values. Put an electron in a conducting band, and its position can take on a set of continuous (not discrete) values. And so then there’s the big question: what about space and time? Are they quantized? Are they discrete? Or might they be continuous, even if there’s a fundamental quantum theory of gravity.
Surprisingly, space and time don’t need to be discrete, but they might be! Here’s what the science has to say so far.
Ask Ethan: If Mass Curves Spacetime, How Does It Un-Curve Again?
“We are taught that mass warps spacetime, and the curvature of spacetime around mass explains gravity – so that an object in orbit around Earth, for example, is actually going in a straight line through curved spacetime. Ok, that makes sense, but when mass (like the Earth) moves through spacetime and bends it, why does spacetime not stay bent? What mechanism un-warps that area of spacetime as the mass moves on?”
You’ve very likely heard that according to Einstein, matter tells spacetime how to curve, and that curved spacetime tells matter how to move. This is true, but then why doesn’t spacetime remain curved when a mass that was once there is no longer present? Does something cause space to snap back to its prior, un-bent position? As it turns out, we need to think pretty hard about General Relativity to get this right in the first place at all. It isn’t just the locations and magnitudes of masses that determine how objects move through space, but a series of subtle effects that must all be added together to get it right. When we do, we find out that uncurving this space actually results in gravitational radiation: ripples in space that have been observed and confirmed.
The deciding results are actually decades old, and were indirect evidence for gravitational waves long before LIGO. Come get the answer today!
If The Universe Is 13.8 Billion Years Old, How Can We See 46 Billion Light Years Away?
“There are a few fundamental facts about the Universe — its origin, its history, and what it is today — that are awfully hard to wrap your head around. One of them is the Big Bang, or the idea that the Universe began a certain time ago: 13.8 billion years ago to be precise. That’s the first moment we can describe the Universe as we know it to be today: full of matter and radiation, and the ingredients that would eventually grow into stars, galaxies, planets and human beings. So how far away can we see? You might think, in a Universe limited by the speed of light, that would be 13.8 billion light years: the age of the Universe multiplied by the speed of light. But 13.8 billion light years is far too small to be the right answer. In actuality, we can see for 46 billion light years in all directions, for a total diameter of 92 billion light years.”
Sure, the Universe is expanding, but how is it possible to see objects that are 46 billion light years away? After all, with an age of 13.8 billion years since the Big Bang, and a Universe where the cosmic speed limit is the speed of light, how can we see light that’s more than three times the expected distance away? It’s one of the most frequent questions that cosmologists get, and yet the root of the question is better framed as “how does the expanding Universe work?” While we normally think about things happening in space that doesn’t change much, or as individual objects moving relative to one another in a static space, we don’t, conventionally, have a solid intuition for how the fabric of space itself expands. But thankfully, the scientists who study it do!
Come learn, in plain English, how we can see so far away in such a young Universe!
Scientists Still Don’t Know How Fast The Universe Is Expanding
“The uncertainties on these two methods are both pretty low, but are also mutually incompatible. If the Universe has less matter and more dark energy than we presently think, the numbers on the ‘leftover relic’ method could increase to line up with the higher values. If there are errors at any stage in our distance measurements, whether from parallax, calibrations, supernova evolution, or Cepheid distances, the ‘distance ladder’ method could be artificially high. There’s also the possibility, favored by many, that the true value lies somewhere in between.”
How quickly is the fabric of space expanding? That depends on how we ask the Universe. If we look at things like the leftover glow from the Big Bang or the large-scale clustering of galaxies, we get a consistent value of 67 km/s/Mpc. But if we look at individual galaxies through a variety of methods, we get a different consistent value: 74 km/s/Mpc. The uncertainties on each method are small and do not overlap, and a potential third way of measuring this (merging neutron stars) have problems and biases all their own. A generation ago, we argued whether the Hubble constant was 50 or 100; the answer turned out to be 70. Now, we argue over whether it’s 67 or 74… or, as a few people propose, that it’s again some value in between the two.
Scientists still don’t know how fast the Universe is expanding. Here’s what the controversy is all about.
The Big Bang Wasn’t The Beginning, After All
“The Universe began not with a whimper, but with a bang! At least, that’s what you’re commonly told: the Universe and everything in it came into existence at the moment of the Big Bang. Space, time, and all the matter and energy within began from a singular point, and then expanded and cooled, giving rise over billions of years to the atoms, stars, galaxies, and clusters of galaxies spread out across the billions of light years that make up our observable Universe. It’s a compelling, beautiful picture that explains so much of what we see, from the present large-scale structure of the Universe’s two trillion galaxies to the leftover glow of radiation permeating all of existence. Unfortunately, it’s also wrong, and scientists have known this for almost 40 years.”
Did the Universe begin with the Big Bang? When we discovered the cosmic microwave background, and its properties matched exactly the prediction of the Big Bang theory, it was a watershed moment for cosmology. For the first time, we had uncovered the origins to the entire Universe, having learned where all of this came from at long last. Emerging from a hot, dense, expanding, and cooling state, the matter-and-radiation-filled early Universe gave rise to everything we see today. Except there were a few pesky problems that the Big Bang couldn’t explain. If the Universe truly emerged from an arbitrarily hot, dense state, and if space and time themselves were born at that exact moment, the Universe would have signatures that we simply don’t see. Instead, theorists came up with an alternative beginning: cosmic inflation. Inflation made a bold prediction about the scale and magnitude of the fluctuations that should arise from this early state, and when our technology finally caught up to our imaginations, we measured them.
It turns out that the Universe didn’t begin from the Big Bang at all. It happened, but it wasn’t the beginning! Find out what came before, and how we know.
Beyond Black Holes: Could LIGO Have Detected Merging Neutron Stars For The First Time?
“We are present at an incredible time in history: at the birth of the observational science of gravitational wave astronomy. The coming decades will reveal a series of “firsts,” and that should include the first binary neutron star merger, the first pinpointing of a gravitational wave source, and the first correlation between gravitational waves and an electromagnetic signal. If nature is kind to us, and the rumors are true, we may have just unlocked all three.”
It seems like an eternity ago, but it’s been under two years since LIGO first began the science run that would first detect merging black holes. Their latest scientific data run is scheduled to end in just two days, and thus far, they’ve announced a total of three black hole-black hole merger discoveries, along with a fourth probable candidate. Yet thanks to the Twitter account of renowned astrophysicist J. Craig Wheeler, a bit of information has leaked: LIGO may have discovered merging neutron stars for the first time. They’d be approximately ten times lighter than the black holes we’ve witnessed merging, which means the signals are only 10% as strong. In order to get the same amplitude, they’d need to be only 10% as distant, cutting the search volume down to 0.1% the volume. But still, neutron stars may be much more abundant, so we might have a chance. Just yesterday, Hubble observed a galaxy with a binary neutron star inside, just 130 million light years away.
Could we have just detected a merging neutron star pair for the first time, in both gravitational waves and electromagnetic radiation, together? The rush is on to find out!
If The Universe Is Expanding, Then Why Aren’t We?
“You might then ask, at what distance does the expansion start to take over? That happens when you average over a volume so large that the density of matter inside the volume has a gravitational self-attraction weaker than the expansion’s pull. From atomic nuclei up, the larger the volume you average over, the smaller the average density. But it is only somewhere beyond the scales of galaxy clusters that expansion takes over. On very short distances, when the nuclear and electromagnetic forces aren’t neutralized, these also act against the pull of gravity. This safely prevents atoms and molecules from being torn apart by the universe’s expansion.”
The Universe is expanding. The farther away a galaxy is, the faster it appears to be receding from us. The standard story tells us that space itself is expanding, and that’s the cause, but it’s only natural to wonder if perhaps space is static, and everything else within it isn’t shrinking instead? Many laypersons choose to go this route, and question the entire field of cosmology as a result. But is this fair? Or is this a road to not only ruin, but to physical inconsistencies? Could we flip the story on its head, and do some sort of test to see if atoms, the planet, or some other ‘local’ entity is shrinking, instead? Or, using the principle of relativity, could we declare that all frames are equally valid, and choose a frame where space isn’t expanding, after all?
These are all interesting thoughts, but there’s only one correct, valid, and consistent way to view the Universe. Sabine Hossenfelder has the explanation!