Time Travel: Observing Cosmic History

The Hubble Space Telescope is many things. It’s an observatory, a satellite, and an icon of cultural and scientific significance – but perhaps most interestingly, Hubble is also a time machine.

Hubble isn’t that far away, locked in a low-Earth orbit just a few hundred miles up that takes about 90 minutes to complete. But with its position just above Earth’s murky atmosphere, Hubble’s transformative view of our universe literally lets us witness our universe’s past.  It allows us to effectively travel back in time.

How does that work? After all, Hubble doesn’t travel beyond our solar system, or even our home planet’s gravity. It certainly doesn’t have any sci-fi elements you might find in Doctor Who or Back to the Future.

Hubble works by gathering light from objects in our universe – some as close as our Moon, and some as distant as galaxy clusters that are billions of light-years away. All that light takes time to reach the telescope, just as it takes time for light to travel from its source to our eyes. For example, our Sun is located about 93 million miles (150 million kilometers) from Earth. That means that it takes roughly eight minutes for its light to reach us here on our planet, so when we look at the Sun (though directly is never recommended!) we see it exactly as it was eight minutes in the past.

Cosmically speaking, the 93 million miles between us and the Sun are nothing. We orbit around just one of billions of stars in the Milky Way Galaxy, which is one of countless trillions of galaxies in the universe.

With that in mind, time travel gets more intense when Hubble observes objects beyond our star system.

Aside from our Sun, the next closest star to us is named Proxima Centauri. It’s about four light-years away, which makes it a close neighbor on a universal scale. But even with Hubble’s sharp, powerful vision, Proxima Centauri remains a point-like object – demonstrating our universe’s unfathomably large size.

Another stellar target of Hubble’s is named Betelgeuse, which is about 700 light-years from Earth. Again, this means that when Hubble looks at Betelgeuse, the star appears exactly as it was 700 years ago. As one of the brightest stars in our sky, astronomers believe it’s likely that even the earliest humans knew of it, as this star appears in stories from several cultures.

This red supergiant star began to dim significantly in the fall of 2019, losing about 60% of its brightness within months. But by April 2020, its regular brightness returned. Hubble studied Betelgeuse and found out that the star “blew its top” – it went through a surface mass ejection, in which the star spewed out a large amount of its surface material into space. When that material in space cooled down, it became a dust cloud that temporarily blocked some of the star’s light.

Hubble’s unique ability to observe in ultraviolet light helped reveal the details of this dimming event and its aftermath. In this range of light, Hubble can better observe the hot layers of atmosphere above a star’s surface.

The telescope continues to be the go-to observatory for scientists who study Betelgeuse. Because it’s taken this long for the light from Betelgeuse to reach us, only in very recent history have we witnessed a cosmic event unfolding that really occurred about 700 years ago!

Scientists also believe that Betelgeuse is on the verge of going supernova – dying in an explosive event. In fact, it may have already done so, but the light from the explosion still hasn’t reached us. There’s a good chance that Betelgeuse no longer exists, though we can still see it today from Earth.

Nebulae are clouds of gas and dust where stars are birthed, or the remnants of a dead or dying star itself. These beautiful, ethereal cosmic objects are the subject of some of Hubble’s most iconic images, but they can also teach us more about how our universe behaves and evolves.

For example, a favorite target for Hubble is the Crab Nebula, located about 6,500 light-years away. There are records from 1054 CE written by Chinese astronomers noting the new presence of a shockingly bright “guest” star in the sky, visible even during the daytime. Turns out, they actually saw a supernova – a star’s explosive death – which became the Crab Nebula, made up by the remnants of this violent event. Of course, those Chinese astronomers witnessed a supernova explosion that occurred about 5446 BCE, but it took the light from the explosion 6,500 years to reach Earth in the year 1054.

When Hubble looks beyond our own galaxy, we can watch cosmic history unfold over eons.

The Andromeda Galaxy is a whopping 2.5 million light-years away, but that’s just the closest major galaxy to us here in the Milky Way. Observing Andromeda is like staring into a vision from 2.5 million years ago – back during the Paleolithic period on Earth, when very early humans existed.

And if Andromeda is the closest major galaxy to us, it’s difficult to comprehend just how far the light from the most distant observed galaxies has traveled.

Another Hubble record is for observing the most distant individual star ever detected, named Earendel. This faraway star emitted its light within the first billion years of the universe, which is about 13.8 billion years old, so it took quite a while to reach Hubble! In fact, that observation was only made possible by nature’s magnifying glass – an astronomical phenomenon known as gravitational lensing. When a massive cosmic object has enough gravity, its gravitational field can magnify and bend light coming from objects located behind it. The gravity of a galaxy cluster located between Hubble and Earendel magnified the star’s light, making it detectable.  The type of star that Earendel seems to be typically have brief lives, only surviving about half a billion years. That means Earendel has ceased to exist for over 12 billion years, yet we are able to look back in time and watch Earendel during its short life.

Perhaps some of Hubble’s most legendary observations are its deep field images, which collect light from thousands of galaxies that are billions of light-years away.

With this type of imagery, we can better understand how our universe changes over time by puzzling out how galaxies evolve. The farther back we look with Hubble, the closer we get to the the big bang, when the universe began – so the most distant galaxies observed by Hubble often appear to us as the “youngest” ones – giving us a sneak peek at the universe in its infancy. Because these galaxies emitted their light when they were young, we get to witness them in their early stages. These early galaxies often appear simpler and smaller than the grandiose spiral galaxies and merged galaxies we see closer to us in distance, and therefore in time. These young galaxies are actually old galaxies now as they have evolved over the time this light has taken to reach us.

Hubble’s farthest observation is of a galaxy named GN-z11, observed as it was 13.4 billion years in the past! This places it within just 400 million years of the big bang itself.

Observations of the most distant objects, like GN-z11 and Earendel, give astronomers exciting insight into the environment of our early universe. The light we see literally traveled from all the way back then!

Our universe remains mysterious, mind-bendingly large, and ever-expanding, but by gathering light from near and far – from the recent past to the dawn of the universe itself – Hubble helps answer questions about where we are and how the universe works.

At its core, astronomy is really just archaeology. Cosmic objects give off light, letting us learn more about their lives. It can take a long, long time for light to reach Hubble – just one telescope orbiting just one planet in just one solar system in just one galaxy. Scientists use Hubble like the time machine that it is to piece together the history and mystery of the cosmos, giving us all a glimpse right up to the edge of the universe – and time itself.