Category: astrophysics

This Is Why ‘Physical Cosmology’ Was Long Overdue For The 2019 Nobel Prize

“It is a spectacular fact of modern science that the predictions of theoretical cosmology have been verified and validated by ever-improving observations and measurements. Even more remarkably, when we examine the full suite of the cosmic data humanity has ever collected, one single picture accurately describes every observation together: a 13.8 billion year old Universe that began with the end of cosmic inflation, resulting in a Big Bang, where the Universe is comprised of 68% dark energy, 27% dark matter, 4.9% normal matter, 0.1% neutrinos, and a tiny bit of radiation with no spatial curvature at all.

Put those ingredients into your theoretical Universe with the right laws of physics and enough computational power, and you’ll obtain the vast, rich, expanding and evolving Universe we have today. What was initially an endeavor of just a handful of people has now become the modern precision science of cosmology. In the middle of the 20th century, legendary physics curmudgeon Lev Landau famously said, “Cosmologists are often in error but seldom in doubt.” With the 2019 Nobel Prize in Physics going to Jim Peebles, perhaps the world will recognize it’s long past time to retire Landau’s quote. We may live in a dark Universe, but the science of physical cosmology has shed a light on it like nothing else.”

I see you out there. You, the person who’s skeptical of dark matter. You, the one who thinks dark energy must be an enormous cosmological mistake. You, who thinks the Big Bang is a hoax and that inflation is a band-aid for a failing theory. And you, especially you, the one who derides cosmology as a pseudoscience, quoting Landau like his more-than-60-year-old quote is still relevant.

Physical cosmology is a real, robust science. It’s not only my field, but my grand-advisor, Jim Peebles, won the 2019 Nobel Prize for his work pioneering it. Come learn what all the fuss is really about.

This Is The One Way The Moon Outshines Our Sun

“Unlike the Sun, the Moon’s surface is made of mostly heavier elements, while the Sun is mostly hydrogen and helium. When cosmic rays (high-energy particles) from throughout the Universe collide with heavy atoms, nuclear recoil causes gamma-ray emission. With no atmosphere or magnetic field, and a lithosphere rich in heavy elements, cosmic rays produce gamma-rays upon impacting the Moon.”

When you view the Moon with your eyes, you’re not seeing it shine so brightly because it’s emitting its own light. Rather, it’s reflecting sunlight on its illuminated phase and reflecting light emitted from Earth (known as “Earthshine”) on the darkened portion. If you look at the Moon in many different wavelengths, from radio to infrared to ultraviolet to X-ray energies, you’ll find that the Sun is much brighter, and the Moon primarily emits light due to reflection.

But in gamma-rays, that entire story changes. The Sun emits virtually no high-energy gamma-rays, with only minor bursts during solar flared. The Moon, on the other hand, emits high-energy gamma-rays constantly; for almost 30 years we know that it outshines the Sun in this particular wavelength range.

It might sound puzzling to you, but there’s a good physics reason for this, and a fun little science fact that everyone should appreciate. Get the story today!

Ask Ethan: How Dense Is A Black Hole?

“I have read that stellar-mass black holes are enormously dense, if you consider the volume of the black hole to be that space which is delineated by the event horizon, but that super-massive black holes are actually much less dense than even our own oceans. I understand that a black hole represents the greatest amount of entropy that can be squeezed into [any] region of space expressed… [so what happens to the density and entropy of two black holes when they merge]?”

The entropy of a black hole, if you simply applied the laws of General Relativity (and nothing else), would simply turn out to be zero. By giving it a quantum description, however, we can get a meaningful formula for entropy: the Bekenstein-Hawking equation. When two black holes merge, the entropy is greater than even the pre-existing entropies combined.

If you think that’s weird, you might suspect that your instinct for density would also be incorrect. Sure, density is just mass divided by volume, but which volume do we use for a black hole? The volume of the event horizon? The volume of a (volume-less) singularity? Something else?

The question of how dense a black hole is has a lot of potential pitfalls, but if we follow the physics closely, we can answer it. Here’s how it’s done.

Did Our Universe’s Structure Grow From The Top-Down Or From The Bottom-Up?

“A century ago, we didn’t even know what our Universe looked like. We didn’t know where it came from, whether or when it began, how old it was, what it was made out of, whether it was expanding, what was present within it. Today, we have scientific answers to all of these questions to within about 1% accuracy, plus a whole lot more.

The Universe was born almost perfectly uniform, with 1-part-in-30,000 imperfections present on practically all scales. The largest cosmic scales have slightly larger imperfections than the smaller ones, but the smaller ones are also substantial and collapse first. We likely formed the first stars just 50-to-200 million years after the Big Bang; the first galaxies arose 200-to-550 million years after the Big Bang; the largest galaxy clusters took billions of years to get there.

The Universe is neither top-down nor bottom-up, but a combination of both that implies it was born with an almost scale-invariant spectrum. With future survey telescopes such as LSST, WFIRST, and the next-generation of 30-meter-class ground-based telescopes, we’re poised to measure galaxy clustering as never before. After a lifetime of uncertainty, we can finally give a scientific answer to understanding how our Universe’s large-scale structure came to be.”

In a top-down scenario, the Universe would form structures on large scales first, then fragment to form individual galaxies. In a bottom-up scenario, the Universe forms tiny structures first, which then collect and clump under their own gravity to bring about a Universe rich in large-scale structure. So, which one is the Universe we have? 

As is often the case, the answer is much more complex than just one of these two possibilities. Come get the full story today.

Three Astrophysicists Reveal Structure Of Universe To Win The 2019 Nobel Prize

“This Nobel is also notable for the elegant way in which it handled a number of controversies. Scientists who work on exoplanets and on large-scale cosmology often compete with one another for funding and resources, but rely on telescopes with similar technologies and often mission-share, as they will with WFIRST and the James Webb Space Telescope. Awarding a Nobel to both cosmology and exoplanets together is a bridge between these two sub-fields, and may encourage them to pursue more joint missions in the future.

Similarly, there were about a dozen Nobel-worthy individuals in the field of exoplanet sciences, with the elephant in the room being that one of the field’s most influential scientists is a known and repeated sexual harasser. In granting a Nobel to Mayor and Queloz, the committee rewarded the exoplanet community while gracefully sidestepping a potential public relations catastrophe.”

The 2019 Nobel Prize in Physics is here, and it goes to three extremely deserving individuals: Jim Peebles, Michel Mayor and Didier Queloz. Mayor and Queloz were the two scientists that, in 1995, unveiled the first confirmed and detected exoplanet around a normal, Sun-like star; it catapulted exoplanet sciences into the mainstream, leading to the rapid development we get to bask in today. Peebles, on the other hand, single-handedly developed the framework for modern physical cosmology, tying observables like galaxy clustering data and CMB fluctuations to the particle properties and energy contents of the Universe.

Peebles also had one student who went on to become a Professor: Jim Fry. That same Jim Fry was my Ph.D. advisor. I believe am the last branch on the Jim Peebles academic tree.

Ask Ethan: Can Gamma-Ray Jets Really Travel Faster Than The Speed Of Light?

“What gives? Is it really possible for gamma-rays to exceed the speed of light and thereby “reverse” time? Is the time reversal just a theoretical claim that allows these hypothetical super-light speed particles to conform with Relativity or is there empirical evidence of this phenomenon?”

Very recently, a paper came out claiming that gamma-ray bursts, and the jets that give them off, can travel faster than the speed of light. If that sounds too fantastic to be true, there’s a reason for that: particles can travel faster than light, but only in a medium, where the speed of light is less than the speed of light in a vacuum. Gamma-ray bursts, when they occur, exhibit a strange property: the signal is mostly a large peak, but when you subtract that peak out, parts of the residual signal are symmetric: if you flip it, part of it going forwards in time is identical to the remainder going backwards in time.

Sound weird? Well, we’re just getting started! Come find out the true story behind this fascinating phenomenon, and what just became our best explanation of what makes it so!

This One Puzzle Brought Physicists From Special To General Relativity

“With an average speed of 47.36 km/s, Mercury moves very slow compared to the speed of light: at 0.0158% the speed of light in a vacuum. However, it moves at this speed relentlessly, every moment of every day of every year of every century. While the effects of Special Relativity might be small on typical experimental timescales, we’ve been watching the planets move for centuries.

Einstein never thought about this; he never thought to calculate the Special Relativistic effects of Mercury’s rapid motion around the Sun, and how that might impact the precession of its perihelion. But another contemporary scientist, Henri Poincaré, decided to do the calculation for himself. When he factored in length contraction and time dilation both, he found that it led to approximately another 7-to-10 arc-seconds of orbital precession per century.“

Special Relativity was easy enough to discover in a certain sense: the Lorentz transformations, Maxwell’s equations, and the Michelson-Morley experiments had been around for decades before Einstein came along. But to go from Special Relativity to General Relativity, incorporating gravitation and the equations governing motion into the same framework, was a herculean effort. However, it was the simple identification and investigation of one puzzle, the orbit of Mercury around the Sun, that brought about Einstein’s new theory of gravity: General Relativity.

What were the key steps, and how did they help revolutionize our view of the Universe? The history is rich and spectacular, and holds a lesson for those on the frontiers of physics today.

One Cosmic Mystery Illuminates Another, As Fast Radio Burst Intercepts A Galactic Halo

“Although scientists have studied [Fast Radio Bursts] intensely since their discovery, their origins remain mysterious. Meanwhile, an estimated 2 trillion galaxies populate our observable Universe. With incredibly large distances for FRBs to traverse, each one risks passing through an intervening galaxy. Giving off multiple pulses of under 40 microseconds apiece, FRB 181112 became the first burst to intercept a galactic halo.”

Where do fast radio bursts come from? Recent studies have demonstrated that they’re associated with host galaxies, but we don’t understand how they work, why some of them repeat, or why the pulse durations are so variable.

What about galactic halos: how much gas is in them? What is the gas temperature, density, magnetization, etc.? These are big questions about galaxies in general that we don’t have a general picture of. If only there were some way to learn more.

How about luck? We got lucky, in November of 2018, when for the first time a fast radio burst passed through a foreground galaxy’s halo. What did we learn? Come get (and see) the full story!

Is The Universe Filled With Black Holes That Shouldn’t Exist?

“What about at the high end of the stellar mass range of black holes? It’s true that pair instability supernovae are real and are indeed a limiting factor, as they don’t produce black holes. However, there’s an entirely separate way to produce black holes that is not particularly well understood at this time: direct collapse.

Whenever you have a large enough collection of mass, whether it’s in the form of a cloud of gas or a star or anywhere in between, there’s a chance that it can form a black hole directly: collapse due to insufficient pressure to hold it up against gravitation. For many years, simulations predicted that black holes should spontaneously arise through this process, but observations failed to see a confirmation. Then, a few years ago, one came in an unlikely place, as the Hubble Space Telescope saw a 25 solar mass star simply “disappear” without a supernova or other cataclysm. The only explanation? Direct collapse.”

As far as our best theories are concerned, the Universe isn’t filled with black holes of all different masses. Instead, the black holes that the Universe forms are inextricably linked to the processes by which the Universe makes the objects that then become black holes. From stars, there’s a theoretical lower limit of about 5 solar masses, and yet we saw a black hole of about 3 solar masses get created. There should be an enormous drop in black hole frequency above about 50 solar masses, but LIGO may be about to challenge that. And even at the highest end, there should be an upper limit to the masses of supermassive black holes, but a few of the ones we’ve found challenge that limit, too.

Does this mean the Universe is filled with black holes that shouldn’t exist? Or does it simply mean that we need superior models? Get the full story today.

Was Dark Matter Really Created Before The Big Bang?

“So if that’s what the observational data points towards, what can we say about where dark matter comes from? A recent headline that made quite a splash claimed that dark matter may have originated before the Big Bang, and many people were confused by this assertion.

It might seem counterintuitive, because the way most people conceive of the Big Bang is as a singular point of infinite density. If you say the Universe is expanding and cooling today, then you can extrapolate it back to a state where all the matter and energy was compressed into a single point in space: a singularity. This corresponds to an initial start time for our Universe — the beginning of our Universe — and that’s the Big Bang.

So how could something that exists in our Universe, like dark matter, have originated before the Big Bang? Because the Big Bang wasn’t actually the beginning of space and time.”

Last month, a paper came out claiming that dark matter may have been created before the Big Bang. Although it might sound implausible, it’s absolutely a possibility that we cannot rule out, although it might be an idea that’s extraordinarily difficult to test when we compare it up against the other options. We have to keep every scenario that hasn’t been ruled out in mind, and understand that despite all we don’t know about dark matter, there’s a ton of indirect evidence brought to us by the full suite of observations at our disposal.

Could dark matter have been created before the Big Bang? Yes, but three other possibilities are maybe even more viable. Come find out why today.