“The reason is simple: with the addition of enough extra free parameters, caveats, behaviors, or modifications to your theory, you can literally salvage any idea. As long as you’re willing to tweak what you’ve come up with sufficiently, you can never rule anything out. If you wanted to concoct a dusty explanation that mimicked the effects of dark energy, you could do it. At some point, though, you lose all physical motivation, and you’re coming up with multi-parameter explanations to explain an observation that a single free parameter — dark energy — gave you before you started tinkering with your dust theory.”
When we look out at the ultra-distant Universe, Type Ia supernovae are our most distant standard candle to work with. From billions or even tens of billions of light years away, we think we know the intrinsic brightnesses of these objects. So measure the apparent brightness, and you know how far away they are, right?
Well, not so fast. What if there’s dust or some other light-blocking phenomenon intervening? Could that mean that these objects are closer than we think, and therefore there’s no need for dark energy? It’s a great idea, and one that we investigated for many years, until the data convincingly showed that no, dust cannot work.
“The question of how quickly the Universe is expanding is one that has troubled astronomers and astrophysicists since we first expansion was occurring at all. It’s an incredible achievement that multiple, independent methods yield answers that are consistent to within 10%, but they don’t agree with each other, and that’s troubling.
If there’s an error in parallax, Cepheids, or supernovae, the expansion rate may truly be on the low end: 67 km/s/Mpc. If so, the Universe will fall into line when we identify our mistake. But if the Cosmic Microwave Background group is mistaken, and the expansion rate is closer to 73 km/s/Mpc, it foretells a crisis in modern cosmology. The Universe cannot have the dark matter density and initial fluctuations 73 km/s/Mpc would imply.
Either one team has made an unidentified mistake, or our conception of the Universe needs a revolution. I’m betting on the former.”
The Universe is expanding: the observations overwhelmingly support that. It’s consistent with Einstein’s General Relativity; it work with the framework of the Big Bang; it allows us to quantify and predict the ultimate fate of our Universe.
But how fast, then, is the Universe expanding?
Scientists can’t agree, because there are three different techniques you can use to measure it. Two agree; one doesn’t.
This Is How The Universe Makes Blue Stragglers: The Stars That Shouldn’t Exist
“New stars form in large clusters, creating stars of all different masses simultaneously.
As they age, the more massive stars die first, leaving only the lower-mass ones behind.
We can date star clusters by examining which stars remain when we plot out stellar color vs. temperature. The older a cluster is, the redder, lower-mass, and less bright its surviving stars are. Globular star clusters are the oldest; some haven’t formed stars in ~13 billion years. Yet if we look closely inside these ancient relics from the young Universe, we’ll find a few blue stars.”
Okay, science fans, I’ve got a mystery for you. When you look at a star cluster, you’ll find a wide variety of stars inside: from the ultra-massive, hot, blue ones down to the lower-mass, cool, red ones. The older a cluster is, the redder it is, because the more massive, hotter, bluer ones burn through their fuel faster and die first. But as a cluster gets redder, we’ll inevitably find a few blue stars that don’t belong. These “blue straggler” stars behave as though they’ve formed at a later time than the rest of the cluster, even though we know that cannot be true. Yet they’re real, they’re there, and their lifetimes are often just 10% the known age of the cluster itself.
Triton, Not Pluto or Eris, Is The Kuiper Belt’s Largest World
“The result, today, is that the largest and most massive body ever to form in the Kuiper belt — 20% larger than Pluto; 29% more massive than Eris — is now Neptune’s largest moon: Triton. Today, Triton makes up 99.5% of the mass orbiting Neptune, an enormous departure from all the other giant planet systems we know of. The only explanation for its properties, especially its bizarre and unique orbit, is that Triton is a captured Kuiper belt object.
We often talk about icy moons with subsurface oceans as candidate worlds for life. We imagine large, distant, icy bodies as planets or dwarf planets in their own right. Triton was born not as a moon of Neptune, but as the largest and most massive Kuiper belt object to survive. You don’t cease to exist when you move locations, and neither did Triton. It’s the original king of the Kuiper belt, and its true origin story is a cosmic mystery that deserves to be solved.”
In October of 1846, just months after Neptune was discovered, a large moon was discovered around it: Triton. Today, Triton is a supremely unusual moon for a number of reasons, but the largest is that it rotates in the wrong direction. While Neptune orbits the Sun counterclockwise and spins counterclockwise on its tilted axis, Triton orbits in the opposite direction. The only way this could have happened is if it were a captured object. And that’s exactly what it looks like: a captured object from the Kuiper belt!
We Just Measured All The Starlight In The Universe, And It Spells Doom For Our Future
“An enormous part of our cosmic history has just been revealed for the very first time. We can bypass the foregrounds of our own Solar System, thanks to these gamma-ray signals and how they interact with the extragalactic background of starlight, to understand and measure how star-formation has occurred over all of cosmic time in our Universe, and to infer the total amount of starlight ever produced.
In the future, scientists may be able to go back even farther, and probe how stars formed and emitted light back before the Fermi-LAT team’s instrumentation is capable of reaching. Star formation is what turns the primordial elements from the Big Bang into the elements capable of giving rise to rocky planets, organic molecules, and life in the Universe. Perhaps, one day, we’ll find a way to reach all the way back to the earliest moments of our Universe, uncovering the truths behind the greatest cosmic mysteries of all. Until then, enjoy each and every step — like this one — that we take along the journey!”
For the first time ever, we’ve measured the total amount of starlight ever produced throughout the history of the Universe. We know how many photons, created by stars, now permeate all of space. We know when star-formation peaked, and we know how it’s fallen over time, and how it continues to fall.
What Was It Like When The First Habitable Planets Formed?
“The galactic center, however, is a relatively difficult place for a planet to be considered habitable beyond a reasonable doubt. Wherever you have stars continuously forming, you have a spectacular slew of cosmic fireworks. Gamma ray bursts, supernovae, black hole formation, quasars, and collapsing molecular clouds make for an environment that is, at best, precarious for life to arise and sustain in.
To have an environment where we can confidently state that life arises and maintains itself, we need for this process to come to an abrupt end. We need something to put a stop to star formation, which in turn puts the kibosh on the activity that is most threatening to habitability on a world. It’s why the earliest, most sustained habitable planets might not be in a galaxy like ours, but rather in a red-and-dead galaxy that ceased forming stars billions of years ago.”
The cosmic story that created the Universe as we know it had a lot of intricate and fascinating steps along the way. The stars needed to live and die to create heavy elements; enough elements needed to form to make life and rocky planets possible; and the Universe needed to quiet down enough in the richest, locations so that life could sustain and thrive. That last step takes surprisingly long relative to the first few! While rocky planets might come into being less than half-a-billion years after the Big Bang, and life might be able to arise in under a billion years, having the right combination of planets that are habitable and continuously hospitable to life might take up to two billion years, even in the most optimistic of circumstances.
Rocky Planets May Only Get Moons From One Source: Giant Impacts
“If your gravity rises up to a point where you can pull yourself into hydrostatic equilibrium — a sphere if you’re static, an ellipsoid if you’re rotating — you cannot be pulled apart by tidal forces so easily. But you could, in principle, develop moons through three methods: initial formation from a protoplanetary disk, capturing another passing body through gravitational forces, or from the debris of a large collision.
While the gas giants display moons that appear to have arisen from all three, the rocky planets, including both major and minor planets, appear to obtain moons from collisions alone. It may be the case that the other options are viable but rare, and we simply have yet to discover them. But following the evidence we have today, perhaps Earth’s moon isn’t atypical after all. Until further notice, giant impacts are the only known way for rocky planets to gain moons.”
When we first visited the Moon and returned samples back to Earth for analysis, scientists were surprised to learn that the lunar surface was practically identical to Earth’s surface. The elements were the same; the isotope ratios were the same; the ages were the same. Unlike the other moons around other planets, the Earth’s moon appeared to be made out of the exact same material as our own world. This helped lead to the giant impact hypothesis as the origin of the Moon. Surprisingly, Mars’ moons and Pluto’s moons appear to have a similar origin: from a giant impact. Of all the planets and dwarf planets with moons, in fact, it looks like giant impacts explain them all.
This Is How Astronomers Solved The ‘Zone Of Avoidance’ Mystery
“From the time of their very first discovery, the Universe’s grand spirals have puzzled astronomers.
While stars, star clusters and other nebulae were concentrated in the plane of our Milky Way, there were no spiral nebulae present. For some reason, they eschewed our galaxy’s plane, which became known as the Zone of Avoidance.”
On the largest scales of all, the one thing we’re certain we can say about the Universe is that it’s extremely uniform. On average, there are the same number of stars and galaxies, the same sized structure, and the same overall density of matter regardless of where we look. So why, then, would we see spiral and elliptical galaxies in roughly equal abundances, in all directions, except within about 10 degrees of the Milky Way’s plane? For some reason, the galaxies that ought to be there simply aren’t; they appear to avoid the plane of the Milky Way.