Our Place in the Universe

October 27, 2015 | Sarah Tse

Nebula - NGC6960
Photo credit: NASA

Answers to profound questions you’ve always wondered about the universe: How old is it? How big? What does it look like?     

Do you ever sit down with your morning cup of coffee, stare introspectively into your bowl of cereal, and ask yourself, “How old is the universe? How big? Where’s the center, and where are we in relation to it?”

Of course you do. Pondering the mysteries of the cosmos every now and then helps to keep your ego in check, and the age, size, and shape of the universe is a particularly humbling prospect. After all, even our own little home planet is already a ripe old 4.54 billion years old. Gauging the approximate age of the Earth was a relatively simple matter of performing radiometric age dating on samples of rocks and meteorite material. The universe is an entirely different kettle of fish.

SEE ALSO: What the Heck is Dark Matter?

Scientists can guess the universe’s age by using its expansion rate, known as the Hubble constant. The expansion itself is caused by dark energy, and the great proportion of dark energy to matter has resulted in an accelerating expansion rate. Combined with the density of matter, researchers can determine how fast the universe expanded and when that expansion began.

To determine the density, composition, and expansion rate of the universe, various missions have measured the residual thermal radiation from the Big Bang, known as cosmic background radiation. These missions performed calculations using those data, and have collaboratively arrived at an age of 13.798 billion years, give or take about 37 million years.

It’s important to keep in mind that the expansion of the universe is a bit different from, say, an expanding balloon. No one could stand outside the universe to see how far it expands, because there is no space or time beyond the universe. Space-time itself is stretching out, carrying with it the matter contained within.

Cosmic background radiation leftover from the birth of the universe.
Cosmic background radiation leftover from the birth of the universe. Photo credit: NASA

We can’t even fathom the true size and shape of the universe because of the constraints of the speed of light. It takes time for light and other signals to reach us on Earth, so the farthest observable limit of the universe is the distance that could have been traveled by light in those 14 billion years. The furthest objects we can see appear as they did right after the Big Bang, and after that, we’re blinded by the light of cosmic background radiation. Expansion also complicates things because in the time it takes for light to reach us from a star’s original location, the star has moved further away.

Our best guess for the size of the observable universe is about 46 to 47 light-years away in every direction—assuming, by the cosmological principle, that the universe behaves under the same forces throughout. That observable universe forms a sphere centered around us, so we are essentially located at the center of our universe.

As time passes, light will reach us from greater distances and the observable universe will grow. But as the universe continues to expand, the objects at the very edges of that growing sphere are moving away from us faster than the speed of light. This means there is a “future visibility limit,” and light emitted from any objects beyond that limit could never enter our observable universe. .

Researchers are still trying to figure out what the universe is like beyond our observable sphere. Einstein’s theory of general relativity, which dictates that matter and energy exert gravitational forces that warp the shape of space-time, continues to reign over these calculations. Physicists have measured the distortions caused by two orbiting galaxies, and found that the universe is both flat and infinite.

An infinite universe will continue to expand, since dark energy doesn’t seem to be running out any time soon. As stars run out of energy and drift further and further apart, everything in the universe could eventually become the same temperature. This daunting fate is called “heat death,” because temperature gradients are necessary to sustain life and other work-intensive processes. That’s assuming that dark energy doesn’t increase so much that it tears apart the fabric of space. Or maybe the whole shebang will implode and collapse into a dimensionless singularity. Nobody knows for sure, and that’s what makes physics both fascinating and terrifying.

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