Category: cosmology

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.

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.

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.

The Expanding Universe Puzzle Just Got Worse, As Incompatible Answers Point To New Physics

“Could there be a problem with our local density relative to the overall cosmic density? Could dark energy change over time? Could neutrinos have an additional coupling we don’t know about? Could the cosmic acoustic scale be different than the CMB data indicates? Unless some new, unexpected source of error is uncovered, these will be the questions that drive our understanding of the Universe’s expansion forward. It’s time to look beyond the mundane and seriously consider the more fantastic possibilities. At last, the data is strong enough to compel us.”

You’ve heard this before, commonly referred to as the “tension” in the expansion rate of the Universe. Two sets of groups are obtaining different values for how fast the Universe is expanding, and the value they get is either close to 67 km/s/Mpc (if you use an early Universe signal) or 73 km/s/Mpc (if you use a late Universe signal). A new result published this week in Science bolsters this, but a reanalysis of the one late Universe signal with a low value (of 69.8 km/s/Mpc) is the biggest deal, as improved calibrations bump that number up by ~4%, enough to put it in line with the other late Universe signals.

If neither the early nor the late group has made a mistake, the true answer is unlikely to lie in the middle. This is why, and here’s what, as a field, astrophysicists need to do about it.

Ask Ethan: Where Is The Center Of The Universe?

“I am wondering how there isn’t a center of the universe and how the cosmic background radiation is [equally] far away everywhere we look. It seems to me that when the universe expands… there should be a place where it started expanding.”

Ah, the old center of the Universe question. If the Big Bang happened a long time ago, and we see galaxies moving away from us faster and faster the farther away they are, then where did the Big Bang happen? Where did the expansion start?

It seems like such a simple question, but it turns out this is the wrong question to be asking. The way space and the expanding Universe works is very different from the picture most of us have in our heads, which is much more like an explosion than like an expansion. Yet there’s a very large suite of evidence that points us away from an explosion.

Instead of asking *where* the Big Bang occurred, we should be asking *when* the Big Bang occurred. It makes a lot more sense when you think about it in those terms. Come and find out why.

If Cosmology Is In Crisis, Then These Are The 19 Most Important Galaxies In The Universe

“In science, different methods of measuring the same properties should yield the same results. But when it comes to the expanding Universe, two sets of groups get consistently different outcomes. Signals from the early Universe yield expansion rates of 67 km/s/Mpc, while late-time signals yield systematically larger values. However, every individual measurement is subject to errors and uncertainties inherent to the method used.”

The strength of any method used in a scientific practice is only as good as the weakest assumption or measurement that’s made. In the case of measuring the expanding Universe, astronomers using an early-time signal get results that are systematically 9% smaller than astronomers using a late-time signal. Of all the late-time signals, the one method with the smallest uncertainties relies on the cosmic distance ladder: tying parallax measurements to Cepheids in the Milky Way, then tying Cepheids to galaxies with Type Ia supernovae, then measuring supernovae everywhere in the Universe. However, there are only 19 galaxies where Type Ia supernovae have been observed that are close enough to have observed Cepheids within them. A tiny statistical fluctuation in the properties of these galaxies could be enough to resolve most or even all of this discrepancy.

It may not be the most likely outcome, but it’s something to keep an eye on. If cosmology is in crisis, then these may be the 19 most important galaxies of all.

We Have Already Entered The Sixth And Final Era Of Our Universe

“In the end, only black dwarf stars and isolated masses to small to ignite nuclear fusion will remain, sparsely populated and disconnected from one another in this empty, ever-expanding cosmos. These final-state corpses will exist even googols of years onward, continuing to persist as dark energy remains the dominant factor in our Universe.

This last era, of dark energy domination, has already begun. Dark energy became important for the Universe’s expansion 6 billion years ago, and began dominating the Universe’s energy content around the time our Sun and Solar System were being born. The Universe may have six unique stages, but for the entirety of Earth’s history, we’ve already been in the final one. Take a good look at the Universe around us. It will never be this rich — or this easy to access — ever again.”

There are a whole slew of events and stages that the Universe has passed through over its cosmic history, and plenty of more to come as the future continues to unfold. But as far as eras of the Universe go, where things make hard transitions from one epoch to another, all of our cosmic history can be divided into six of them. From inflation to the primordial soup of the hot Big Bang to the plasma-rich early Universe to the cosmic dark ages to the stellar age to the dark energy era, our entire natural history fits nicely within these boxes.

The only existential problem? The entirety of Earth’s existence has occurred in this sixth and final era. We’re already in the end stages; see how far we’ve come and learn how far we’ll go!

Sorry, Black Holes Aren’t Actually Black

“If you have an astrophysical object that emits radiation, that immediately defies the definition of black: where something is a perfect absorber while itself emitting zero radiation. If you’re emitting anything, you aren’t black, after all.

So it goes for black holes. The most perfectly black object in all the Universe isn’t truly black. Rather, it emits a combination of all the radiation from all the objects that ever fell into it (which will asymptote to, but never reach, zero) along with the ultra-low-temperature but always-present Hawking radiation.

You might have thought that black holes truly are black, but they aren’t. Along with the ideas that black holes suck everything into them and black holes will someday consume the Universe, they’re the three biggest myths about black holes. Now that you know, you’ll never get fooled again!”

So, you thought you knew all there way to know about black holes? That if you get enough mass together in a small enough volume of space, you create an event horizon: a region from within which nothing can escape, not even light. So how is it, then, that black holes wind up emitting radiation, even long after the last particle of matter to fall into them has ceased?

There are two ways this occurs, and both are completely unavoidable. Black holes aren’t actually black, and this is how we know it.