February 2024

Understanding the evolution of the Universe — using light as a time machine

One of the most fundamental and profound discoveries made by humankind is how the Universe began in the “Big Bang” and how it has evolved. These discoveries have been made in large part by the detection of visible and infrared light, with most of today’s leading astronomical instrumentation using detectors produced by Teledyne.

Finite speed of light allows us to “look back in time”

While light propagates extremely fast – 299,792 km per second (186,282 miles per second) – the finite speed of light means that when we observe a distant object, we see the state of the object when the light left the object, not the state of the object today. The fact that light does not travel infinitely fast is extremely valuable for studying the evolution of the Universe. By looking at objects at different distances, we can see how the Universe evolved over time. In this way, we use light as a time machine. The further we look in distance, the further we see back in time.

As shown in the figure above, light from the Sun takes 8 minutes to reach Earth, light from Pluto takes 4 hours to reach Earth, and light from the closest star to our Sun, Proxima Centauri, takes 4.2 years to reach Earth. The Andromeda Galaxy, which is a relatively close galaxy in our local group of galaxies, is 2.5 million light-years from Earth. If we see a supernova explode in Andromeda we are witnessing something that happened 2.5 million years ago.

The limit to how far we can see is not due to the size of the Universe; the limit to how far we can see is set by how long the Universe has existed – 13.8 billion years, when the Universe began with the “Big Bang”.

The distant universe is an infrared universe

A very important thing to know about light propagation from the distant Universe is that as the Universe expands, the wavelengths of light propagating through the universe get stretched. When stars are formed, they emit light in the ultraviolet and visible. But light emitted in the UV and visible from the first stars and galaxies has been stretched so that when their light reaches the Earth, the light is “redshifted” into the infrared.

To study the distant universe, we need to observe it in infrared light.

A concise history of the universe and the motivation for the James Webb Space Telescope (JWST)

The primary motivation for JWST is to improve our understanding of the history of the Universe. The figure below (right) presents our best understanding of how the universe evolved. It is impossible to depict the huge range of scales for time and size, so this figure is a visual representation with size shown on the vertical axis and time on the horizontal axis.

This figure is for the portion of the Universe that we can see. The “observable universe” is the portion of the Universe that has been able to propagate light to us since the time of the Big Bang.

The theory of the universe is a fantastic story. As of launching of JWST in December 2021, the consensus understanding of the evolution of the early universe was as follows.

13.7 billion years ago, the observable universe started as a quantum fluctuation. A very small volume of space, less than one-millionth the size of an electron, went through exponential expansion to become the size of soccer ball in 10^-32 of a second. This ball was pure energy, far too hot for particles to form, with a temperature of 10²⁷ to 10²⁸ K (degrees Kelvin).

After 1 second, the observable universe had expanded to 20 light-years in diameter. The expansion cooled the universe enough so that sub-atomic particles could form, particles like quarks and gluons. This environment, 10,000 times hotter than the center of the Sun, is what the CERN particle collider in Switzerland is recreating, temperature of 10¹⁰ K.

After two minutes, the observable universe had expanded to be 300 light-years wide. The universe had cooled down enough so that protons and neutrons could be formed. At this time, the observable universe was a hot plasma, a gas of charged particles where light and matter are coupled.

The universe kept expanding for the next 380,000 years, until when the observable universe was 80 million light-years wide, it was cool enough to allow hydrogen and helium to form and for light to break free of matter. This is the “first light” that we can see in the universe. It was an orange glow when it was emitted. Since that time, the universe has expanded by a factor of 1000 and we see this “first light” now as the Cosmic Microwave Background (CMB). The CMB, which has a 1.0 mm wavelength, was discovered in 1964 by Arno Penzias and Robert Wilson at Bell Labs in Holmdel Township, New Jersey.

At about 380,000 to 400,000 years after the Big Bang, the universe stopped creating new light and entered the “Dark Ages”. It took 100 to 250 million years for gravity to pull material together and stars to form and new light to be generated. The first light of stars and galaxies is what JWST is designed to see.

• The reader should be aware that JWST has observed some early galaxies that are larger and better formed than the existing theory would predict. We eagerly anticipate the enrichment of our understand of the universe from JWST’s observations.

The rapid expansion of the early universe is explained by the theory of “inflation”. It is important to understand that during the inflationary phase, matter was not travelling faster than the speed of light. Space itself was expanding.

The Hubble Space Telescope (HST) cannot be used to see back to the first light from the first stars and galaxies because of three main reasons:

1. The HST is a warm telescope
• The HST primary mirror is heated to 70 degrees Fahrenheit since it was not fabricated to go to cold temperatures without distortions. Infrared light emitted by its optics will overwhelm faint distant signals.
2. The HST detectors only can see out to 1.7 µm.
• 1.7 µm is not far enough into the infrared to measure the early universe.
3. The HST aperture is only 2.4-meters (8 feet) in diameter.
• Astronomers need a larger aperture to collect light from the very faint early objects. The 6.5-meter diameter aperture of JWST has 7 times the light collecting area of HST.
• The larger JWST aperture also provides nearly 3 times finer angular resolution.

A comparison of HST and JWST is shown below. HST is in low Earth orbit at an altitude of 569 km (353 miles), orbiting the Earth every 97 minutes. To block light from the Earth, Moon, and Sun, the HST primary and secondary mirrors are shielded by the long optical tube. The primary differences between HST and JWST are shown in the table below.

	Hubble Space Telescope	James Webb Space Telescope
Primary Mirror diameter	2.4 meters	6.5 meters
Primary Mirror temperature	20⁰C (293 K)	-223⁰C (50 K)
Total Mass	12,437 kg	6,200 kg
Optimized for wavelengths in	Visible	Infrared
Orbital Location	Low Earth Orbit	Lagrange Point 2