Section 2.7 - Unsolved Astronomical Questions

There are a number of unsolved questions in astronomy; these are just a few that I find interesting!

The Antimatter Puzzle

Why is there more matter than antimatter? In fact, why is there any matter at all? 

Known more formally as "baryon asymmetry," neither the standard model nor general relativity is able to explain why the Universe is made up almost exclusively out of matter when the Big Bang should have produced equal amounts of matter and antimatter. Huge amounts of matter and antimatter were created in the first moments of the Universe, but most of it collided and annihilated immediately. What's left in the Universe is a tiny fraction of matter that remained as a result of some sort of imbalance

One might argue - perhaps we're living in a region populated with matter, but other regions are populated by antimatter. What's the problem with this argument? If this were true, we'd observe huge bursts of gamma rays from the boundaries where those two regions meeting and matter and antimatter start to collide

Physicists are experimenting with different exotic matter-antimatter particles to try find any imbalances in their properties that could've tipped the balance in favor of matter winning out 

Dark Matter

Why do galaxies and galaxy clusters rotate at faster speeds than what physics should allow without flying apart?

We'll learn Kepler's Laws in the orbital mechanics section, but essentially, a collection of matter has a center of gravity, and the escape velocity needed to pull away from that center of gravity is directly proportional to the amount of mass. For example, if the Moon suddenly sped up significantly, it would escape Earth's gravity and fly away! Keplerian mechanics accurately describes orbits as large as that of the Solar System, but galaxies defy this relation

We're able to estimate the mass of a galaxy, but we're seeing stars in the arms of galaxies that are rotating too fast for the observed mass and should be ejected from the galaxy. There must be some invisible mass that adds to the gravity that keeps the galaxy together

We call this unknown source of gravity dark matter not because it's "dark" in the normal sense of the word, but because we haven't been able to detect it. It must be some sort of particle that doesn't interact with any of the four fundamental forces except gravity

Dark Energy

The collective gravity of the Universe should be slowing down the expansion observed by Hubble. So why is it speeding up?

The fact that the Universe's expansion is accelerating is as ludicrous as the idea of throwing a ball up in the air and watching it accelerate upward faster and faster away from Earth's gravity. But this is exactly what we observe. Our only hypothesis is that there must be some sort of thus far undetectable energy that overpowers gravity on the large scale and repels everything. This sounds strangely similar to the cosmological constant Einstein forced into his field equations to counteract the force of gravity and maintain a static Universe. Perhaps the greatest mind of the 20th century will get the last laugh in the 21st?

Regardless, according to our calculations, it seems that what we see may only be a small fraction of what's really out there

Exoplanets and Extraterrestrial Life

Are we alone in the Universe?

This is a question that countless generations of humanity have pondered, and no one has any definitive answer yet despite constant searching and attempts to contact aliens. 

The Drake equation is a hypothetical equation that quantifies the number of intelligent alien civilizations we should be able to communicate with, based on a number of factors. 

The problem is we have no way of getting realistic numbers to input. Everything is just wild speculation. As we finance people like to say when it comes to Microsoft Excel financial modeling, "garbage in, garbage out." And more importantly it begs the question, "if advanced alien life exists, why hasn't it contacted us?"

Despite our failure to find alien life, we've been very successful in detecting exoplanets, planets that orbit other stars. Detecting them is incredibly difficult, like trying to see a pebble next to a bonfire light years away. The first exoplanets were discovered in the 1990s, and to date we've detected almost 4,000 of them, primarily through three detection methods (described in more detail here):

  • Detection Method 1 - Radial Velocity:
We tend to think of stars as stationary relative to their planets, but they do in fact feel a little tug from their planets as well. This little tug draws the star towards and away from us at regular intervals, causing red and blue shifts that we can detect

  • Detection Method 2 - Transit Photometry
A planet passing in front of a star dims it ever so slightly. This can be confused with sunspots, but if it happens at regular intervals its probably a planet

  • Detection Method 3: Gravitational Microlensing
Recall from the theoretical physics unit how Einstein predicted gravitational distortions could amplify light when something massive passes in front of something else. If the first object to pass by is a star, and then the star is followed by a planet, we should observe a second temporary amplification in the light of whatever is behind it. 

The Fate of the Universe

What will eventually become of the Universe indefinitely in the future?

The fate of the Universe depends primarily on the density of matter that propagates it. 

  • The Big Freeze - If there isn't enough mass (gravity) to reverse the Universe's expansion, the the Universe will expand forever and temperature will continue to decrease until all the stars burn out. Whether this expansion continues to accelerate or levels out depends on both mass and whatever the true nature of dark energy is 
  • The Big Crunch - If there is enough mass, then the Universe will eventually reverse and contract back into a single point. In fact, if this is the case, it's possible the true nature of the Universe is a repeating cycle of Big Bangs and Big Crunches

This video is by far the best animation of the future of the Universe I've found. It's long, but it's worth it!