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.
Why Cosmology’s Expanding Universe Controversy Is An Even Bigger Problem Than You Realize
“The question of how quickly the Universe is expanding is one that has troubled astronomers and astrophysicists since we first realized that cosmic expansion was a necessity. While it’s incredibly impressive that two completely independent methods yield answers that are close to within less than 10%, the fact that they don’t agree with each other is troubling.
If the distance ladder group is in error, and the expansion rate is truly on the low end and near 67 km/s/Mpc, the Universe could fall into line. But if the cosmic microwave background group is mistaken, and the expansion rate is closer to 73 km/s/Mpc, we just may have a crisis in modern cosmology.
The Universe cannot have the dark matter density and initial fluctuations that such a value would imply. Until this puzzle is resolved, we must be open to the possibility that a cosmic revolution may be on the horizon.”
Ever since we first learned that the Universe was expanding, scientists have worked hard to measure just how fast that expansion rate is. From that, combined with what makes up the Universe, we can learn how old the Universe is and what it was like in the past, as well as what it’s fate will be in the future. Yet the two groups that make independent measurements of that rate, from the cosmic microwave background and the cosmic distance ladder, have gotten inconsistent results. If the distance ladder team has made a mistake, everything will be fine with cosmology. But if that team is right and the microwave background team is wrong, there should be a crisis coming.
Why is that? Come find out why the biggest controversy in modern cosmology might be an even bigger problem than almost everyone realizes!
Einstein Wins Again! General Relativity Passes Its First Extragalactic Test
“For the first time, we’ve been able to perform a direct test of General Relativity outside of our Solar System and get solid, informative results. The ratio of the Newtonian potential to the curvature potential, which relativity demands be equal to one but where alternatives differ, confirms what General Relativity predicts. Large deviations from Einstein’s gravity, therefore, cannot happen on scales smaller than a few thousand light years, or for masses the scale of an individual galaxy. If you want to explain the accelerated expansion of the Universe, you can’t simply say you don’t like dark energy and throw Einstein’s gravity away. For the first time, if we want to modify Einstein’s gravity on galactic-or-larger scales, we have an important constraint to reckon with.”
For many of the greatest cosmic puzzles today, you can either add two new ingredients, dark matter and dark energy, or you can seek to modify Einstein’s theory of gravity. While Einstein’s General Relativity has been confirmed spectacularly under a wide variety of circumstances, the only robust tests that are independent of dark matter or dark energy assumptions occur on scales of the Solar System or smaller. That’s only for distances that are a tiny fraction of a light year, and for masses no bigger than the Sun, which should trouble you when you’re making inferences about galaxies, clusters, or the entire Universe! But thanks to a very fortunate galactic system
a strong gravitational lens that is only 500 million light years distant
we’ve been able to put Einstein’s theory of gravity to the test for galactic masses and distance scales in the thousands of light years.
Was there ever any doubt that Einstein would win again? Here’s what happened, and here’s what it means for alternative theories of gravity!
The Counterintuitive Reason Why Dark Energy Makes The Universe Accelerate
“In a nutshell, a new form of energy can affect the Universe’s expansion rate in a new way. It all depends on how the energy density changes over time. While matter and radiation get less dense as the Universe expands, space is still space, and still has the same energy density everywhere. The only thing that’s changed is our automatic assumption that we made: that energy ought to be zero. Well, the accelerating Universe tells us it isn’t zero. The big challenge facing astrophysicists now is to figure out why it has the value that it does. On that front, dark energy is still the biggest mystery in the Universe.”
There are lots of explanations out there for why the Universe’s expansion is accelerating. Some people point towards the negative pressure of a cosmological constant and talk about how this causes space to fly apart. Others call it a “fifth force” and imply that it’s a new fundamental relation that functions as some sort of anti-gravity. Neither of those explanations are correct, though, and they both complicate a much simpler (and more correct!) truth: that the Universe’s expansion rate is simply determined by all the different types of matter and energy within it. Dark energy is just another type of energy, but it’s different in a very particular way from the normal matter, dark matter, neutrinos, and radiation that we know.
Dark energy makes the Universe accelerate because of how it evolves and changes differently from everything else we know of over time. Come find out how!
NASA’s Next Flagship Mission May Be A Crushing Disappointment For Astrophysics
“This is NASA. This is the pre-eminent space agency in the world. This is where science, research, development, discovery, and innovation all come together. The spinoff technologies alone justify the investment, but that’s not why we do it. We are here to discover the Universe. We are here to learn all that we can about the cosmos and our place within it. We are here to find out what the Universe looks like and how it came to be the way it is today.
It’s time for the United States government to step up to the plate and invest in fundamental science in a way the world hasn’t seen in decades. It’s time to stop asking the scientific community to do more with less, and give them a realistic but ambitious goal: to do more with more. If we can afford an ill-thought-out space force, perhaps we can afford to learn about the greatest unexplored natural resource of all. The Universe, and the vast unknowns hiding in the great cosmic ocean.”
While the Trump administration just proposed a new branch of the military, a “space force” if you will, NASA has just demanded that every one of the proposed astrophysics flagship missions abandon their large ambitions and present a scaled-down, sub-$5 billion version of their proposal. That means smaller telescopes, reduced capabilities, and less knowledge that will be revealed about the Universe. Every single one of the four will suffer from this, but the biggest losers may be us. In terms of science, society, spinoffs, and civilization, we’ll all be poorer if we fail to invest in something that truly makes a difference in this world.
Why grandstand when you can literally grandly stand where no human has stood before: at the frontiers of knowledge? It’s time to invest in something that matters.
Meet The Universe’s First-Ever Supermassive Binary Black Holes
“In 1891, the object OJ 287, 3.5 billion light years distant and a blazar itself, optically bursted. Every 11-12 years since, it’s produced another burst, recently discovered to have two, narrowly-separated peaks. Its central, supermassive black hole is 18 billion solar masses, one of the largest known in the Universe. This periodic double-burst arises from a 100-150 million solar mass black hole punching through the primary’s accretion disk.”
The big problem with black holes is that, well, they’re so dark. They don’t emit any detectable light of their own, so we have to rely on indirect, secondary signals to infer their existence. That usually arises in the form of radio and X-ray radiation from matter that gets accelerated by the black hole’s extreme gravity, as well as from the magnetic fields that an accretion disk around the black hole can create. The radiation can form jets, and when a jet points at our eyes, we see a blazar. Well, the system OJ 287 has a periodic blazar that flares in a double-burst every 11-12 years, indicative of a large, supermassive black hole orbiting an even more massive behemoth, punching through the accretion disk twice with every orbit.
Come meet OJ 287, first found to burst way back in 1891, and still one of only two supermassive black hole binaries known in the Universe!
Ask Ethan: How Do We Know The Age Of The Solar System?
“How do we know the age of our solar system? […] I have a loose grasp on the concept of dating the time elapsed since a rock was liquid, but 4.5 Billion years is roughly how long ago Theia hit proto-Earth liquefying a massive amount of everything. […] How do we know we’re actually dating the solar system and not just finding dozens of ways to date the Theia collision?”
You’ve probably heard the estimates before: that the Earth, the Sun, and the rest of the Solar System are all about 4.5 or 4.6 billion years old. But why be so imprecise? We don’t have to be! In fact, we know that there are slight variations, and based on the fact that we think that the Earth-Moon system formed from a giant impact tens of millions of years after the rest of the Solar System did, we shouldn’t get the same answer for everything! It turns out that we’ve now advanced to the point where we can actually give answers that are extremely accurate: the Earth-Moon system should be 4.51 billion years old; the oldest meteorites show an age for the rest of the Solar System of 4.568 billion years, and the Sun may be a little older at 4.6 billion years.
How do we know? The science of radioactive decay holds the answer, and it’s a lot more complex, but a lot more well-understood, than you might think!
Mars Opportunity And Spirit Rovers Could Have Lived Practically Forever With One Tiny Change
“If one extra piece of equipment, such as a compressed air blower aboard a robotic arm, were installed, dusty solar panels could be cleaned at will. Hunkering down to survive a dust storm, even one that blocked 100% of the light, wouldn’t be catastrophic so long as the rovers had enough power stored in their batteries to control and operate the blower mechanism. Had that been in place, Spirit could have saved itself from its 2010 fate, and Opportunity wouldn’t be in the danger it’s in now, in the midst of the enormous dust storm it’s experiencing. Still, even though hindsight is 20/20, it’s pretty hard to be sad about two missions that overachieved beyond anyone’s expectations. But for next time, it’s an invaluable lesson: if you can protect yourself from Martian dust accumulation, you could potentially live forever. At least, if you’re a rover on Mars.”
If a dust storm blots out the Sun, then we shall rove in the shade, says the brave Mars rover. But for a rover like Opportunity, which relies on solar panels, this is a lousy, battery-draining strategy that would be its death knell. Despite the fact that it’s lasted for over 5,000 Martian days and roved for over 45 kilometers, this single large dust storm that it’s caught it could be its absolute end. Unless a natural cleaning event occurs, its panels may be so dust-covered as to be useless, which is how Spirit, its twin, met its demise in 2010. Although the rover has far exceeded its expectations, if it were built with the capability of actively addressing the dust accumulation problem, both Spirit and Opportunity could have lived, practically, forever.
Here are the options for the tiny changes that could have been made that would have kept them alive indefinitely. Go, little rover, go!
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.
The Surprising Reason Why Neutron Stars Don’t All Collapse To Form Black Holes
“The measurements of the enormous pressure inside the proton, as well as the distribution of that pressure, show us what’s responsible for preventing the collapse of neutron stars. It’s the internal pressure inside each proton and neutron, arising from the strong force, that holds up neutron stars when white dwarfs have long given out. Determining exactly where that mass threshold is just got a great boost. Rather than solely relying on astrophysical observations, the experimental side of nuclear physics may provide the guidepost we need to theoretically understand where the limits of neutron stars actually lie.”
If you take a large, massive collection of matter and compress it down into a small space, it’s going to attempt to form a black hole. The only thing that can stop it is some sort of internal pressure that pushes back. For stars, that’s thermal, radiation pressure. For white dwarfs, that’s the quantum degeneracy pressure from the electrons. And for neutron stars, there’s quantum degeneracy pressure between the neutrons (or quarks) themselves. Only, if that last case were the only factor at play, neutron stars wouldn’t be able to get more massive than white dwarfs, and there’s strong evidence that they can reach almost twice the Chandrasekhar mass limit of 1.4 solar masses. Instead, there must be a big contribution from the internal pressure each the individual nucleon to resist collapse.
For the first time, we’ve measured that pressure distribution inside the proton, paving the way to understanding why massive neutron stars don’t all form black holes.