How far back in time can we see?

Carolyn Devereux, University of Hertfordshire

How far back in time can we see?

The Hubble and James Webb Space Telescopes have observed the most distant star ever seen – Earendel – whose name means morning star. Even though Earendel is 50 times the mass of our sun, and millions of times brighter, we would not normally be able to see it. We can only see it due to an alignment of the star with a large galaxy cluster in front of it whose gravity bends the light from the star to make it brighter and more focused. The galaxy cluster essentially acts as a lens.

Astronomers see into the deep past when we view distant objects. Light travels at a constant speed (3×10⁸ meters per second). So, the farther away an object is, the longer it takes for the light to reach us. By the time the light reaches us from very distant stars, the light we are looking at can be billions of years old. Thus, we are looking at events that happened in the past.

When we observe Earendel’s light, we are looking at light the star emitted 12.9 billion years ago. We call this the lookback time. That’s just 900 million years after the Big Bang. But because the universe has also expanded rapidly in the time it took this light to reach us, Earendel is now 28 billion light-years away from us.

Hubble’s successor, the James Webb Space Telescope, may be able to detect even earlier stars. However, they would have to be nicely aligned to form a gravitational lens so that we can see them.

Webb can see back further in time

To see farther back in time, the objects need to be very bright. And the farthest objects we have seen are the most massive and brightest galaxies. The brightest galaxies are ones with quasars in them. Quasars are luminous objects powered by supermassive black holes.

Before 1998, the farthest detected quasar galaxies were about 12.6 billion years lookback time. The improved resolution of the Hubble Space Telescope increased the lookback time to 13.4 billion years, and with the Webb we expect to improve on this possibly to 13.55 billion years for galaxies and stars. (Read about Webb’s early record-breaking galaxy discoveries.)

Stars started to form a few hundred million years after the Big Bang, in a time that we call the cosmic dawn. We would like to be able to see the stars at the cosmic dawn, as this could confirm our theories on how the universe and galaxies formed. That said, research suggests we may never be able to see the most distant objects with telescopes in as much details as we like. The universe may have a fundamental resolution limit.

Looking back to before there were stars

One of the main goals of Webb is to know what the early universe looked like and when early stars and galaxies formed, thought to be between 100 million and 250 million years after the Big Bang. And, luckily, we can get hints about this by looking even farther back than Hubble or Webb can manage.

We can see light from 13.8 billion years ago, although it is not starlight, because there were no stars then. The farthest light we can see is the cosmic microwave background. The cosmic microwave background is the light left over from the Big Bang, forming at just 380,000 years after our cosmic birth.

The universe before the cosmic microwave background formed contained charged particles of positive protons (which now make up the atomic nucleus along with neutrons) and negative electrons … and light. The charged particles scattered the light, which made the universe a foggy soup. As the universe expanded, it cooled until eventually the electrons combined with the protons to form atoms.

Unlike the soup of particles, the atoms had no charge, so the light didn’t scatter and could move through the universe in a straight line. This light has continued to travel across the universe until it reaches us today. The wavelength of the light got longer as the universe expanded … and we currently see it as microwaves. This light is the cosmic microwave background. We can see it uniformly at all points in the sky. The cosmic microwave background is everywhere in the universe.

Seeing beyond the cosmic microwave background?

The cosmic microwave background light is the farthest back in time that we have seen. We cannot see light from earlier times because that light was scattered and the universe was opaque.

There is a possibility, however, that we can one day see even beyond the cosmic microwave background. To do this, we cannot use light. We will need to use gravitational waves. These are ripples in the fabric of spacetime itself. If any formed in the fog of the very early universe, then they could potentially reach us today.

In 2015, scientists using the LIGO detector detected gravitational waves from the merging of two black holes. Maybe the next generation space-based gravitational wave detector – such as LISA, due for launch in 2037 – will be able to see into the very early universe before the cosmic microwave background formed 13.8 billion years ago.

Carolyn Devereux, Senior Lecturer in Astrophysics, University of Hertfordshire

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Bottom line: How far back in time can we see? Earendel is the farthest star we have seen, and the cosmic microwave background reveals the first light in the universe. But can we see beyond that?