4.2 The Future of Cosmology

summary

Although we understand much about the past 13.8 billion years, many mysteries still remain.

Scientists often ponder the ultimate origin, the ultimate fate, and the true composition of our universe.
While inflation can answer many questions about our cosmic origins, we still need to test it experimentally, study its physics in detail, and determine what—if anything—happened before it.

The fate of our universe depends largely on what exactly it’s made of.

~95% of cosmic matter is not atoms, but dark energy (~68%) and dark matter (~27%). A better understanding of these phenomena would aid in determining the ultimate destiny of our universe.
Five “cosmochalypse” scenarios are examined in the lecture—Big Chill, Big Crunch, Big Rip, Big Snap, and Death Bubbles. The expansion rate of the universe is what determines which of these would happen.
The expansion rate depends entirely on the density of the universe, as governed by the Friedmann equation:$$H^2=\frac{8 \pi G}{3}\rho$$ where $H$ is the Hubble parameter, $G$ is Newton’s gravitational constant and $\rho$ is the density.

One way to study dark energy is to measure the red-shift and the brightness of the billions of luminous objects that adorn our universe.

The brightness can tell us about the object’s distance while the red-shift infers the object’s receding velocity.
Plotting the velocity as a function of distance should yield a linear pattern as governed by the Hubble parameter.
Since we can see these objects many different times in the past, we are able to measure exactly how fast their part of the universe was expanding at different stages in the cosmic history.
From this, we can infer exactly what the density of dark energy was at various times in the past to see if it remained constant.

Dark matter can be studied in a variety of ways.

Weigh it—This is how dark matter was first discovered. We weigh cosmic bodies by examining gravitational forces. Gravitational lensing can also be used by exploiting the fact that gravity bends light.
Catch it—Dark matter particles are extremely “shy” and hardly ever interact with particles we commonly see. But we might be able to detect dark matter when it bounces off another particle, similar to how neutrinos are detected.
Infer it—Fermi bubbles at the center of the Milky Way can possibly be explained by densely-packed dark matter particles bumping into each other, annihilating, and producing Gamma rays.
Make it—The Large Hadron Collider at CERN might be able to create dark matter particles at super high-energy collisions.