Photons: The Wanderers in Space after the Big Bang


By Emily LevesqueUniversity of Washington

Just after the Big Bang, our universe looked very different than it does today. If we could go back in time and look around, we’d see, well, not much. The universe in those early times was incredibly hot and opaque, filled with a miasma of free electrons that blocked the path of any photon, or particle of light, that was trying to travel.

Illustration of the Big Bang explosion
After the Big Bang, photons weren’t able to travel far since they were surrounded by a sea of free electrons. (Image: FTCjuan08/Public domain)

The Big Bang Was Crowded

Photons couldn’t easily travel across the universe; instead, they would bounce around in this sea of free electrons like someone trying to push through a crowded room. However, according to the Big Bang theory, the universe would be rapidly expanding at this point in the aftermath of the Bang itself, cooling off as it went.

After about 300,000 years the universe cooled off to a chilly 3,000 Kelvin—colder than the surface of the Sun, but still hotter than any modern-day blast furnace—allowing the electrons to team up with protons and make the universe’s first atoms. 

Since this tied up the electrons, the universe became less opaque and photons were free to go rocketing from one end of the still-rapidly-expanding universe to the other. If we could go back in time to this moment, we’d see these photons appearing and beginning their trip across the universe as a faint, deep red glow.

The Blackbody Radiation Principle

We know that these photons would look deep red thanks to a principle known as blackbody radiation. If a theoretically ideal body that absorbs all electromagnetic radiation is warmed, the laws of thermodynamics tell us that the blackbody will emit energy and that the energy it emits will be directly related to the object’s temperature. 

This energy is emitted as photons, the elementary particles that make up light. Photons emitted by a warm object will carry away very specific and discrete amounts of energy, measured by scientific instruments in terms of different wavelengths of light, which we would perceive as different colors.

We can write out the relationship between a photon’s energy, wavelength, and speed using the equation: E equals h times c over lambda. E here refers to energy, while c refers to the photon’s speed—the speed of light—and lambda refers to the photon’s wavelength. To us, longer wavelengths look red, while shorter wavelengths look blue. 

The only thing left to account for in this equation is that little h; this is Planck’s constant, named for Max Planck. Max Planck was a German physicist who, in the late 19th century, made groundbreaking discoveries about blackbody radiation, mathematically describing how photons work and how warm objects emit them.

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Max Plank to the Rescue, Again

You can see how blackbody radiation works—and why we know those photons traveling through the universe after the Big Bang would have appeared deep red—if you play with this equation.

Blackbody radiation tells us that, since Planck’s constant and the speed of light both stay the same, if we raise the value on the left side of this equation—the energy—we have to lower the value of the wavelength in the denominator to keep the equation balanced. 

This means the wavelength has got shorter; in other words, a higher-energy photon will look bluer. What does this have to do with temperature? For that, we need another discovery of Max Planck’s, this one known as Planck’s law. 

Diagram showing the blackbody radiation curve
The area under the blackbody radiation curve of something tells us how much energy it’s emitting. (Image: Darth Kule/Public domain)

He was able to mathematically write out how the energy emitted by a blackbody would depend on its temperature. The algebra of this equation tells us that if we raise an object’s temperature, we raise the amount of energy that it emits and decrease the wavelengths of light that carry away most of that energy.

Comparing Stars

This is a spectrum, plotting wavelength on the horizontal axis and brightness on the vertical axis. A hot object—say, something like our Sun, with a temperature of about 6,000 Kelvin—will emit light like this according to Planck’s law. This is the Sun’s blackbody radiation curve. 

The area under this curve tells us how much energy the Sun is emitting, and the shape of the curve tells us how much energy the Sun is emitting at different light wavelengths. Most of the Sun’s energy is being emitted as light with a wavelength of about 500 nanometers, which we perceive as yellowish-green light!

Image of Sirius in the night sky
Sirius is the brightest star in the night sky. (Image: NASA/Public domain)

Let’s pick Sirius, the brightest star in the night sky with a temperature of nearly 10,000 Kelvin. When we use the Planck equation to draw out its blackbody radiation curve, we get a curve with much more area under it, which tells us that the star is emitting more energy, and a curve that peaks at a much shorter wavelength; this is why, to us, Sirius appears as a brilliant blue-white star.

A Very Long Trip Indeed

Finally, let’s consider our 3,000 Kelvin temperature in the early universe, and the light emitted in the early universe that was beginning to travel through space. Its blackbody curve has less area under it—so we know it’s emitting less energy—and it peaks at a longer wavelength, emitting most of its light as infrared photons. The light near these wavelengths would appear as a faint deep red glow to the human eye.

These infrared photons that began speeding through space 300,000 years after the Big Bang are still traveling through the universe, and some of them are just now arriving here on Earth! Those photons, however, have been dramatically redshifted thanks to the expanding universe. 

We can also think of this as the radiation from the early universe “cooling off,” corresponding to a much colder blackbody temperature today of only 2.7 Kelvin (that’s minus 455 degrees Fahrenheit!) If we go back to the laws of thermodynamics and blackbody radiation, that means the photons we detect from that era should have much longer wavelengths, observable as microwaves.

Common Questions about Photons

Q: Why couldn’t photons travel easily in the universe immediately after the Big Bang?

Immediately after the Big Bang, photons would have to bounce around in a sea of free electrons and couldn’t travel freely.

Q: What is blackbody radiation?

If a theoretically ideal body that absorbs all electromagnetic radiation is warmed, the blackbody will radiate energy, the amount of which is directly related to the object’s temperature.

Q: What do the photons that started traveling at the start of the universe look like when they finally reach Earth now?

They have been redshifted since the Big Bang and the expansion of the universe. Also, the law of thermodynamics and blackbody radiation indicate that these photons should be observable as microwaves due to their long wavelength.

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