What is Farthest Away?
Articles,  Blog

What is Farthest Away?

If you’ve ever looked at the night sky,
you’ve probably wondered, what’s out there? And how far does it extend? Well … you’re in good company. For thousands of years, humans have been curious
about what’s off the edge of the map. Today, science may finally be close to giving
us answers to even bigger questions: Does the map even have an edge? And what’s the farthest away anything can
be? [OPEN] When humans first started mapping their location
in the world around the 6th century BC, the edge of the map was… not very far away. Our earliest world maps showed flat, circular
views of the Earth, like you’d draw from a mountaintop…if the Earth was flat… which
it’s not. In fact, the Greek scholar Eratosthenes figured
out the Earth was basically spherical around 240 BC. He used the midday sun to calculate the circumference
of the Earth and got results within 15% of the actual value (40,075 km). But still, for centuries, maps were flat,
centered around whoever drew it, whether they were in Europe, the Arab world, or even China. At the edges, mapmakers would often inscribe,
“here be lions” to indicate unknown lands… not “here be dragons” like I’ve always
believed. Explorers continued to exchange knowledge
and “discover” places that people already lived, and our picture of the world expanded. Our world map was finally complete with the
discovery of Antarctica in the 1820s. Suddenly, the edge of the map became the sky
itself. In the 1830s, we discovered that stars are
vastly farther away than planets–duh. We figured this out by measuring how different
astronomical objects shift relative to their background–a phenomenon called parallax. The edge of our map now extended beyond our
solar system to the galaxy. Of course, by the 1900s we still thought our
Milky Way galaxy was the whole universe. Here’s the challenge astronomers faced:
If one star is dimmer than another, is it farther away? Or is it just smaller? In the early 1900s, astronomer Henrietta Swan
Leavitt realized a class of stars called Cepheids pulsate, bright and dim, and every Cepheid
that pulsates at the same rate has the same brightness. If two Cepheid stars are pulsating at the
same rate, and one’s dimmer than the other, that means one is actually farther away. And since there were pulsating Cepheid stars
whose actual distances we already knew, this let us build a cosmic measuring stick. In 1925, when Edwin Hubble looked at the brightness
of variable stars in a smudge called Andromeda, they appeared much dimmer than their pulsing
rate predicted, and Hubble realized these stars were too far away to be in the Milky
Way. Andromeda was another galaxy. We now know that light from Andromeda, which
on a dark night is actually the most distant thing you can see with your naked eye. Using bigger eyes–like the giant space telescope
named for Hubble, we’ve seen images in which every speck is its own galaxy, the farthest
of which is more than 13 billion light years away, almost the oldest light we can see. When we look at a galaxy like this one, we
see it as it was when the universe was just 400 million years old…before Earth or our
Sun even existed yet! We also don’t see distant galaxies where
they currently are. Certain stars, like our sun, emit light in
characteristic patterns, depending on their atomic makeup. These “absorption spectra” are like a
luminous fingerprint. But if we see that pattern at longer, redder,
wavelengths than we expect, we know the object is actually moving away from us, its light
has been stretched by the expanding universe. Astronomers call this redshift. So in the time light takes to reach us from
a distant galaxy, it’s moved farther away. So, when we look at a distant galaxy, we’re
seeing as it was when it was much younger, and much closer. Imagine your friend sent you an email from
a train saying she was just crossing the bridge leaving town. By the time the email reaches you, she’s
no longer on the bridge, and probably already farther away. Here’s where the physics gets really freaky/weird. Since it was given off, the light from a distant
galaxy like GN-z11 traveled more than 13 billion light years to get to us, but that galaxy
today is almost 46 billion light years from Earth. And it turns out, the further we look at distant
objects, the faster they are moving away from us. This means there’s some distance, way out
there, beyond which galaxies are receding faster than the speed of light, because space
itself is expanding that fast. The light they give off will always be moving
away from us. We can never see that far. That’s the limit of our observable universe. Beyond that limit, we’re nearly certain
there are other parts of the universe whose light we will never see. This means no one really knows how big the
actual universe is. Some say it’s at least 250 times bigger
than our observable universe, others calculate it could exceed 10^10^10^1222 megaparsecs,
which is a vast and complicated and… ridiculous number. Others say there is no edge at all. There’s one edgeless theory that says the
the fabric of the universe may be folded to up like a torus ­— a fancy donut shape. That would mean not only no edge to our cosmic
map, but no beginning or end to it. What is farthest away may not be a question
that we can answer, however, we’re driven by the same curiosity that drove those very
first mapmakers, thousands of years ago. Just that their maps were a bit smaller. Stay curious, and be careful… there be lions out there.


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