3.2 Searching Supernovae

summary

Not every supernova will provide cosmologists with a standard candle.

There is a special kind of supernova known as a Type Ia supernova. These were first explained by the Indian astrophysicist Chandrasekhar in the 1930s.
The core of a white dwarf star (which is about the same mass as our Sun) can only hold back the crushing force of gravity to a certain critical mass, known as the Chandrasekhar mass. It does so with a principle known as electron degeneracy pressure.
If another nearby star begins to spill material onto the white dwarf, little by little it will accumulate mass until it crosses the Chandrasekhar limit, causing a runaway thermonuclear explosion. The star crushes and fusion occurs throughout it immediately.
This explosion’s brightness, at its peak, is about 4 billion times the luminosity of our Sun. That means we can see them from incredibly far away—halfway to three-quarters of the way across the visible universe, using the Hubble Space Telescope

How do we search for Type Ia supernovae?

These types of explosions can be very rare, making this a “needle-in-a-haystack” problem. There will only be one such supernova in a given galaxy every hundred years or so. Modern technology allows us to monitor tens or hundreds of thousands of galaxies at once, enabling astrophysicists to see many supernova events each year. Computers calculate the difference in brightness of two pictures taken at a time interval apart, which pinpoint the supernovae
In 1994, the High-z Supernova Search Team was formed by Brian P. Schmidt, with the intent to measure the cosmic deceleration of the universe, using Type Ia supernovae. It was expected that the degree of deceleration would reveal the mass and fate of our universe.
By 1997, the team had collected the first set of supernovae to analyze. Adam Riess designed a computer program to calculate the mass of the universe using the equation $$q_0 = \frac{\Omega_M}{2}-\Omega_{\Lambda}$$where $q_0$ was the deceleration, $\Omega_M$ was the mass, and $\Omega_{\Lambda}$ was the cosmological constant, which was neglected.
The mass should have either been a very small number (like 0.3) or a big number (like 1). Instead, it was a negative number.

A negative number led to an incredible discovery.

There’s no such thing as a negative mass. The solution had to be that the deceleration rate was negative, meaning the universe’s expansion wasn’t slowing down at all—it was speeding up.
There had to be something causing this acceleration, something like Einstein’s cosmological constant. Not only would it have to be acting in this equation, it would have to be the dominant force—much stronger than the attractive force of gravity.
In 1998, Adam’s team published the paper “Observational Evidence From Supernovae For An Accelerating Universe and a Cosmological Constant,” where they showed that the universe was about 70% in the form of dark energy.
A competing team, the Supernova Cosmology Project (led by Saul Perlmutter), came to exactly the same conclusion. This rapidly because the breakthrough of the year.

Why do we think the universe is accelerating now?

We still don’t truly understand the physics of dark energy, but we have a few good guesses as to its nature.
It’s possible it is what we refer to as the vacuum energy, or the energy of empty space. Quantum physics tells us that empty space is actually a sea of virtual particles, which could cause the repulsive gravity in Einstein’s theory. However, there is a large disconnect between what our measurements of this value tell us and what the energy should be to account for dark energy.
It could be that we have a dynamical dark energy—a new, transient energy that changes from place to place and from time to time. Just like the electric or magnetic field, it would fill space and have a value at all places.
Or, it could be that we still don’t fully understand gravity itself. Maybe we have the wrong theory of physics; maybe dark energy, dark matter, and other mysteries will be solved with a readjustment of our fundamental theories?

Another worry existed about the measurements.

What if when we were looking at distant supernovae, where we thought “faint” meant distant, our measurements were obscured by something like dust in the universe?
If we could look out infinitely far, the dimming effect from dust would go on forever. But a dimming effect from dark energy would stop after about 5 billion light years. And so astrophysicists looked to measure supernovae beyond this 5 billion light year mark.
In 2002, astronauts installed a new advanced camera on the Hubble Space Telescope, allowing the High-z team to observe 25 of these distant supernovae from an earlier era of the universe, when it was still decelerating and before it gave way to dark energy. Their measurements confirmed the new paradigm—the universe is now expanding ~20% faster than it was 5 billion years ago.
This isn’t the only observation that convinced scientists the universe is filled with dark energy. There are now several independent lines of investigation that all give us the same picture.