Ask Ethan: How can we look back in time until the Big Bang?

Our cosmic history of the universe, not contrary to the best observations and theories for today

Time moves forward, and the past will not return. From the point of view of man, we call it the arrow of time: the past is just a memory; the future has not yet come, and all that we can experience is the present. It is assumed that everything in the Universe submits to this property, and all interactions either occurred in the past, or occur now, or occur in the future. But does this mean that the past should only become a memory for the universe? Our reader is concerned about the fact that in fact everything is not quite like this:
How do we see photons of relic radiation, if the Earth did not exist at the time when they were emitted? Shouldn't these photons have run away from us into our future?

This idea is difficult to realize: we declare that we are looking into the past for billions of years, but how exactly do we do this if even Earth didn’t exist for a long time?

Protoplanetary disk around a young star in the artist's view

Uncovering the history of our solar system is a bit like a detective story: we only have evidence from those that remained and survived until today, and we need to recreate the rest of the story of how we got to this moment. People’s records go back a maximum of several thousand years - and before that we only have evidence of a biological, chemical, geological, and physical history. We can recreate the history of life on Earth thanks to an understanding of DNA, evolution, fossil remains, radioactive decay, coal deposits, etc. We can recreate the history of the solar system by studying the myriad of planets, moons, comets and asteroids available to us. Thanks to the indirect evidence available to us, we learned a lot about how the Earth came to its present state.

The massive collision of large planetesimals gave rise to the Earth / Moon system, and we learned about this by flying to the Moon and returning its surfaces to the Earth.

Earth exists only 4.5 billion years old - less than a third of the history of the universe. And we can only guess about our past, but not observe it directly. But someone, sitting at a sufficiently large distance from us, could observe our past directly. Why? Because for them - this is the present.

View of the Earth and the Moon from Cassini in Saturn orbit, July 19, 2013. The image of Earth is about 67 minutes younger than it was for us at the time of creating the photo.

If you looked from the Moon to the Earth, you would see the Earth as it was 1.3 seconds ago, since the light takes about 1.3 seconds to make such a journey. If you were on Pluto, you would see the Earth as it was less than 5 hours ago. But to truly appreciate how the past Earth was different from the present, you could only at more serious distances:
• With Proxima Centauri, the closest star to the Sun, you would see the Earth as it was 4.2 years ago.
• From Sirius, the brightest star in the sky, you would see the Earth as it was 8.6 years ago.
• From Rigel, the brightest blue star in the constellation of Orion, you would see the Earth as it was 773 years ago.
• With Deneb, the farthest of visible bright stars, you would see the Earth as it was 2600 years ago.
• From Andromeda, the closest galaxy to the Milky Way, you would see the Earth as it was 2.2 million years ago.
• With Messier 84, one of the most distant galaxies in the Virgo cluster, you would see the Earth as it was 60 million years ago, soon after the extinction of dinosaurs.
• With IC 1101, the largest known galaxy in the universe, you would see the Earth as it was 1.05 billion years ago.
• With GN-z11, the most distant galaxy known to us, you would see the Earth as it was 13.4 billion years ago.

Of course, 13.4 billion years ago, there was no Earth - perhaps the Milky Way did not exist then! You would see what was there at that time - matter, which eventually turns into the Milky Way, stars, planets, one of which - after another 9 billion years - will form into the Earth.

For us, the laws of physics work in the same way as for those located somewhere else. And when we look at all these distant stars or galaxies, we see the light that they emitted millions and billions of years ago. This light changed over time: the universe expanded, and the wavelength of light increased. The brightest ultraviolet from the most distant galaxies stretched so much that it passed from the ultraviolet, through the entire visible part of the spectrum, and turned out to be in the infrared part. There are probably galaxies beyond the capabilities of our infrared telescopes, since their light has shifted to a longer wavelength part of the spectrum that is inaccessible to the Hubble telescope infrared camera.

If we are set up decisively enough, we can look for signs of the Big Bang itself, outside of any galaxy. In the early time stages, the universe was filled with a sea of ​​matter, antimatter, and radiation particles. Over time, matter and antimatter annihilated, and left a small amount of excess matter, and the radiation wavelengths stretched due to the expansion of the universe. Since the wavelength and energy are connected - the longer the length, the less energy - the Universe cools with expansion, which means that at some point we reached an important stage: electrons and protons began to form neutral atoms that the radiation no longer split into . At this moment, the radiation begins to travel freely, smoothly and straightforwardly.

Today we see that this radiation took 13.81 billion years to travel to us. When we look at the Universe and see relic radiation, we see a light that:
• appeared during the Big Bang,
• last interacted, scattered from a free electron at the last moment, when the Universe was filled with free electrons,
• traveled 13.81 billion years in an expanding universe
• came to us and hit the detector, moving into the microwave part of the spectrum after this amazing journey.
The Big Bang light increases the wavelength over time, loses energy and density, but still does not go away; you just need to know how to find it.

This light will really fly past our eyes, but always, at any moment of the future, a new light will appear, from more distant points of the Universe, which will reach our eyes for the first time. It will be an even colder light, from earlier times, with a lower photon density. In 100 billion years it will not be microwave, but radio emission, thanks to the Universe continuing to expand. But the further we look, the more part of the Universe will open to us.

And someone, located as far away from us, will not see the Earth or the Milky Way, looking in our direction - but only the light from the Big Bang, exactly the same as we see, looking in their direction.


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