Astronomical Distances and the Age of the Universe

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Podcast Transcript

Every so often, astronomers will publish photos taken with an astronomical telescope and say that the object they captured is so many billions of light years away. 

But how could they know the distance of something from just looking at it? 

Furthermore, astronomers claim that the universe is almost 14 billion years old. How could they possibly know that?

Well, there are answers to these questions, and surprisingly, astronomical distance and the age of the universe and closely intertwined. 

Learn more about astronomical distances and the age of the universe on this episode of Everything Everywhere Daily.

In a previous episode, I talked about radiometric dating and how researchers can determine the age of plants and birds and rocks and things. 

What all of these techniques have in common is that they use the known rate of decay of radioactive elements as a type of clock to determine the age of something. 

If you look at the original isotope’s ratio to the element it decays into, you can get an idea of how old something is. 

The key to this method is that you need a sample of the thing that you want to test.  When it comes to astronomy, especially very distant objects, we don’t have any samples. All we have are observations and measurements of the light. 

By a similar token, just because astronomers can see an object, how can they tell how far away something is? 

As it turns out, the answer to the age of the universe is tied up in the question of how distant astronomical objects are. 

To get the answer to these questions, we need to climb something called the Cosmic Distance Ladder. 

Basically, there are different methods that can be used to determine various distances. 

Let’s start with something pretty easy: the moon. 

We can measure the distance to the moon with an incredible amount of accuracy by using the speed of light.  

Light has a fixed speed when it travels through the vacuum of space. The fundamental constant is the key to almost every one of the methods I’ll be talking about because at astronomical distances, light is all we have to work with. 

To determine the distance to the moon, you can just bounce light or a radio signal off the moon and determine how long it takes to come back. You divide that number by two and then multiply by the speed of light to get the distance. 

The speed of light is approximately 300,000 kilometers per second or 186,000 miles per second. 

There were specially designed reflectors that were sent to the moon during the Apollo missions, which are still functioning. Using these devices, you can measure the distance to the moon within centimeters. 

This technique can also be used to bounce radar waves off of other bodies in our Solar System.

What about objects further out, like stars? 

To measure stars that are reasonably close to the Earth, and by close, I mean about 10,000 lightyears, you can use a method known as parallax. 

Parallax basically uses trigonometry to determine distances by measuring angles and a known distance.

If you want to experience a version of parallax, you can do it right now. Hold your thumb out in front of your face with only one eye open. Notice what your thumb is covering up. Now only look through your other eye.  Trying doing this really fast, just looking through one eye and the other.

You’ll notice that what your thumb is covering up is slightly different. This is because your eyes are slightly apart and looking at your thumb from a slightly different angle. 

That is parallax. 

With astronomy, instead of knowing the distance between our eyes, we can use observations at different times of the year, when the Earth is at one point in its orbit and six months later when it is at the opposite point in its orbit. 

For stars within the right distance, they will appear to move slightly against the background of stars, just like your thumb appears to move as you switch your eyes. 

If you can measure the angle of the object’s apparent change and know the diameter of the Earth’s orbit, you can then calculate the distance to the object. 

This has a limit because the farther away something gets, the angle gets narrower and narrower. 

If the angle is one arcsecond, which is 1/3600 of a degree, then the distance is defined as one parsec, equal to 3.26 light years. 

This is extremely handy information to know if you are planning to do the Kessel Run. 

With better telescopes, like the Hubble or the Webb telescope, you can get measurements down to 20 to 40 microarcseconds, which allows you to measure up to 16,000 lightyears. 

10,000 to 16,000 lightyears only really let us measure our corner of the galaxy, which is 100,000 lightyears across. 

What about objects further away in the Milky Way? How can we measure the distance to those? 

For that, we need something called a standard candle. 

A big problem is that you can’t tell how far away a light source is by just looking at the light. It could be a very bright light that is far away or a dimmer light that is closer to you. They would appear exactly the same.

If you know the absolute brightness of something, called its luminosity, then you can measure its observed brightness and determine the difference. 

Once you know the difference between the luminosity and the observed brightness, you can calculate the distance with a pretty simple formula. 

The most common standard candle is a special type of star known as a Cepheid variable. Cepheid variable stars have a brightness that pulsates. It turns out that the luminosity of the star is tightly correlated to the period it takes to pulsate. Measure the period of the pulse, you get the luminosity, you can measure the apparent brightness here on Earth, and you got your distance.

If you remember back to my episode on supernovas, some of them have unique properties beyond just being a giant explosion. A type 1a supernova only explodes once it has accreted exactly the right amount of mass from a neighboring star. 

This is known as the Chandrasekhar limit The fact that all type 1a supernovas explode with the exact same amount of mass means that they have the exact same luminosity. Assuming you can find one, you can then measure its distance.

Depending on the standard candle which is used, this can take you use this method to measure object throughout the entire Milky Way and potentially even some nearby galaxies. 

What about the images which are being taken by the Webb and Hubble Telescopes, which claim to show objects from the edge of the observable universe? How can we measure those distances?

For that, we need something known as Hubble’s Law.

Hubble’s Law is named after the early 20th-century astronomer Edwin Hubble who discovered one of the most important fundamental facts in astrophysics. 

Galaxies move away from Earth at speeds proportional to their distance.

In other words, the farther away a galaxy is, the faster it is moving away from us. 

How do we know that galaxies are moving away from us, and how can we know that more distant ones are moving faster? 

This has to do with what is called the redshift. The red shift is nothing more than the doppler effect applied to light. 

You are probably familiar with the doppler effect when it comes to sound. A car will increase its pitch as it approaches you and decreases its pitch as it moves away. 

This is because sound is a wave, and when a wave moves towards you, it is compressed, and when it moves away, it is extended. 

In the case of light, instead of changing pitch, it changes color. Objects moving away will have longer wavelengths of light, shifting towards the red part of the spectrum. 

Objects moving towards you have shorter wavelengths and are shifted to the blue part of the spectrum. 

Because stars are made of hydrogen, remember back to my previous episode on stars, we know the exact spectrum given off by hydrogen when it glows. By looking at the hydrogen spectrum, you can see how much it was shifted and measure its velocity.

Hubble’s law states the velocity of a galaxy equals its distance times a constant. That constant is known as the Hubble Constant. 

The units of the Hubble Constant are kilometers per second per megaparsec.  So, for every million parsecs a galaxy is away, it will be traveling a set number of kilometers per second faster. 

The current best estimate for the Hubble Constant is 67 kilometers per second per megaparsec. 

You might have noticed that kilometers and megaparsecs are both units of distance, and they would actually cancel each other out if you convert megaparsecs to kilometers, leaving the actual unit of the constant as the reciprocal of seconds. Seconds being a unit of time. 

So far, all I’ve been doing is talking about measuring distances, but at the beginning of the episode, I was talking about the age of the universe. This is the point where they come together. 

Do a little bit of algebra moving the variables around, and you get a value expressed as a unit of time. 

That time is the age of the universe. 

A great deal of time and effort has been put into getting better and better values for the Hubble Constant. The most recent effort, called the Planck Collaboration, measured the Cosmic Microwave Background radiation to get a measurement. 

The value they came up with gave an age of the universe of 13.787±0.020. 

There have been other estimates as well, and they are all within about 30 million years of each other. 

This value has been refined over the years, and I would expect it to get refined even further as more advanced telescopes come online. 

All the methods I’ve mentioned for determining astronomical distances get more accurate the closer the object is to Earth, which should be expected. 

So, whether or not you realize it, the size of the universe is intrinsically tied up to the age of the universe. It is all due to Hubble’s Law which is one of the most elegant equations in all of science. 

So the next time you hear something on the news about an astronomical discovery billions of light years away, know that there is a method to the madness, and there are techniques developed over the last century which can measure both the size and age of the universe.

Everything Everywhere Daily is an Airwave Media Podcast. 

The executive producer is Darcy Adams.

The associate producers are Thor Thomsen and Peter Bennett.

Today’s review comes from listener IrishEnglishTeacher over at Podcast Republic. They write:

I teach ESL and often recommend the podcast to my students. I would like to point out that in Ireland, we still use “ye” to distinguish between you singular and plural, and the accusative (object pronoun) is her, not she. I am a member of the completionist club. Keep up the good work.

Thanks, IrishEnglishTeacher! In the course of researching the episode on Shakespearian English, I did come across the fact that in Yorkshire, they do use a form of Thou, as do some Quakers who speak in a dialect called plain speak. 

However, I didn’t come across anything about the Irish use of Ye, which is really interesting. I never heard it in my various trips to Ireland, but then again, I wasn’t really in a situation where Ye would have been used. 

Remember, if you leave a review or send me a boostagram, you too can have it read on the show.