Hubble has shown us some of the universe’s earliest galaxies and defined the limits of their age. Its vision has uncovered evidence of black holes and discovered the mysterious, unexplained phenomenon called dark energy.
When we look out at space, we are looking back in time. The light arriving at our location from the farthest objects in the universe is light that left those objects billions of years ago, so we see them as they appeared long ago.
So what do we see, when we capture the light from these farthest objects? The most distant galaxies look strange – smaller, irregular, lacking clearly defined shapes.
No telescope before Hubble had the resolution to see these distant galaxies. Intrigued, astronomers turned Hubble on what appeared to be a nearly empty patch of sky and let it soak up all the light it could for 10 days. They were taking a risk – most Hubble observations take just hours, and the time being eaten up could have been used for more concrete needs. It was possible the objects the astronomers were looking for would be too faint or small for even Hubble to see.
But the results turned up a treasure trove: 3,000 galaxies, large and small, shapely and amorphous, burning in the depths of space. The stunning image was called the Hubble Deep Field.
In subsequent years, Hubble teamed with other observatories to examine small patches of the sky in high resolution, long exposures, and multiple wavelengths.
The deeper Hubble sees into space, the farther it gazes back in time. This chart illustrates the regions that have fallen under Hubble’s eye.Enlarge Image
Surveys like the Hubble Ultra Deep Field (HUDF) and the Great Observatories Origins Deep Survey (GOODS) have provided pictures of vast, deep collections of galaxies – including some that existed when the universe was less than a billion years old.
The images allow us to follow the development of the universe. Tiny red dots -- early, shapeless galaxy building blocks whose light has been stretched by the expanding universe into an infrared glow – litter the most distant parts of the visible universe. Closer in, we see numerous galaxy interactions and collisions as galaxies come together and merge, growing in the process. And nearer still, we see versions of the large, stately galaxies we know today.
Measuring Distances with Cepheids
The answer to the age of the universe is beaming down on us from the sky. We know the universe has been expanding since the Big Bang, so if we can measure its size and its expansion rate, we can extrapolate the age of the universe.
It's harder than it sounds. Since you can't extend a ruler out into the stars, all estimations are made by studying objects’ brightness. Cepheid variable stars are a special type of pulsing star whose cycles of intensity and dimness indicate their inherent brightness. When astronomers find Cepheid variable stars in galaxies, they compare how bright they truly are with how faint they appear over distance, and thus determine the distance to those galaxies. It's something like judging the distance to a car on a dark road by gauging the brightness of its headlights.
Before Hubble, astronomers had only been able to narrow the universe’s age down to 10-20 billion years old – not a particularly exact measurement with 10 billion years of leeway.
Hubble performed the definitive study of 31 Cepheid variable stars, helping to determine the current expansion rate and thereby narrow the age of the universe down to the most accurate it's ever been. Its observations of Cepheid variable stars in galaxies like NGC 4603, combined with measurements by other observatories, eventually pinned the age down to 13.7 billion years old, plus or minus a few hundred million years. Hubble’s observations helped change the age of the universe from a vast range of possibilities to the kind of number whose precision required a decimal point.
Knowing the age of the universe isn't just a matter of curiosity. By giving us a time scale for the development of stars and galaxies, it helps us refine our models of how the universe – and everything in it – formed.
Quasars shine from both single (left) and colliding galaxies (right) in these images. The galaxies are 1.5 and 1.6 billion light-years away, respectively.
When astronomers first turned radio telescopes on the sky, they tracked radio wave sources to some typical cosmic objects, including the remains of supernovae, distant galaxies, and powerful areas of star birth. But one particular type of object looked like nothing more than a point of light, perhaps a star. Further observations showed that these objects were extremely far away, meaning they could only be distant galaxies. The objects, called "quasars," were thought to be the incredibly bright centers of those far-away galaxies.
The distance to quasars is so great, and their actual size so small – about the size of our solar system -- that the mere fact that we can see them via telescope makes quasars the brightest objects we've discovered in the universe. In fact, one of Hubble’s contributions to the quasar mystery was to prove with its high resolution there actually was a galaxy hidden behind the glare.
A supermassive black hole creates a jet of particles, traveling at nearly the speed of light from the center of galaxy M87. The jet bursts forth from the disk of material swirling around the black hole.
Hubble observations also helped determine that these brilliant galactic centers are powered by giant black holes. As matter falls into a supermassive black hole, the surrounding region heats up and releases tremendous amounts of energy and light, creating a quasar. Hubble found quasars in the centers of galaxies that are colliding or brushing up against one another, as well as in elliptical galaxies, which are thought to have developed as a result of multiple galactic mergers. These interactions may help "feed" the black hole and light up the quasar.
And it wasn't just quasars. Hubble found that almost all galaxies with bright, active centers have supermassive black holes feeding off the galaxy's matter. Further, the mass of the black hole is related to the mass of the bulge of stars around the center of the galaxy, indicating that the formation of a galaxy is closely connected to the formation of its black hole.
Possible fates of the Universe
Our universe started with a bang and has been expanding ever since, the space between galaxies increasing with time. For many years, astronomers contemplating the death of the universe considered two main possibilities: either the universe would go on expanding forever, the galaxies gently drifting apart; or the universe would stop expanding and fall back on itself in a "big crunch."
Discovering Dark Energy
Certain types of exploding stars, called Type 1a supernovae, always give off about the same amount of light. This makes the brightness of these explosions nearly identical.
By the late 20th century, astronomers were confident the answer was that the universe would expand forever at a constantly decreasing speed, coasting like a car out of gas but never quite running out of momentum. They began working to determine the rate of the expansion.
The brilliant stellar explosions known as supernovae could be used to find the distances to galaxies and thus measure the expansion of the universe. Hubble's clear vision allowed astronomers to find extremely distant supernovae. But Hubble’s observations threw the standard assumptions into disarray: the universe wasn't slowing down at all. Examining the properties of the supernovae Hubble had imaged, astronomers found that the universe was speeding up as though something were propelling it, driving its expansion faster and faster.
What is it? Scientists still don't know. Some suspect a previously unknown "dark energy" is the culprit. Hubble continues to study the supernovae that could be the key to solving this mystery.