Edwin Hubble, Doppler Shift, and the Expanding Universe

Summary

Professor Dave explores Edwin Hubble's pivotal role in astronomy, detailing how Hubble's categorization of galaxies and discovery of their redshift led to the understanding of an expanding universe. This revelation, along with the Doppler effect's explanation of light wavelength changes, supports the Big Bang theory. Hubble's law, correlating galaxy distances with their recession velocities, further cements this cosmological model, offering a profound insight into the universe's evolution.

Takeaways

• 🌌 Edwin Hubble played a crucial role in understanding galaxies and categorizing them.
• 🔭 Telescopes in the 20th century revealed that nebulae are actually distant galaxies.
• 🌍 Our perception of the universe expanded from a solar system to realizing that the Milky Way is just one galaxy among billions.
• 🚀 Hubble observed that almost all galaxies are redshifted, meaning they are moving away from us.
• 🎈 The expanding universe is compared to a balloon where all points (galaxies) are moving away from one another.
• 💥 Hubble's findings laid the foundation for the Big Bang theory, suggesting that the universe started from a single point.
• 📉 The farther a galaxy is, the faster it is moving away, known as Hubble’s law.
• 🔵 The Doppler effect explains how the shift in light frequency reveals a galaxy's motion: blue shift when moving toward us and red shift when moving away.
• ⚛️ Emission spectra help determine the motion of galaxies by analyzing the shifts in hydrogen and other elements.
• 📏 Hubble's law uses the Hubble constant to relate the recession velocity of galaxies to their distance from us.

Q & A

Who is Edwin Hubble and what is his contribution to astronomy?

Edwin Hubble was an American astronomer who made considerable contributions to the field of astronomy. He is best known for developing the categories and subcategories for classifying galaxies and for providing observational evidence that the universe is expanding. His work led to the formulation of Hubble's Law, which describes the relationship between the distances and recessional velocities of galaxies.

What was the significance of the realization that many objects in the night sky are not part of our galaxy?

The realization that many objects visible in the night sky are actually separate galaxies, not part of our Milky Way, expanded our understanding of the universe significantly. It revealed that our galaxy is just one of billions of galaxies, each containing billions of stars, thus making the universe far larger and more complex than previously thought.

What is the Doppler effect, and how does it relate to the observation of galaxies?

The Doppler effect is a phenomenon where the frequency of waves changes based on the relative motion of the source and the observer. In the context of galaxies, the Doppler effect causes a shift in the observed wavelengths of light emitted by galaxies. If a galaxy is moving away from us, its light will be redshifted (shifted towards the red end of the spectrum), and if it is moving towards us, it will be blueshifted (shifted towards the blue end).

How did Edwin Hubble determine that galaxies are moving away from each other?

Edwin Hubble determined that galaxies are moving away from each other by observing the redshift in their emission spectra. He found that almost every galaxy in the observable universe exhibited redshift, indicating that they are moving away from us. This observation, along with the exception of nearby galaxies gravitationally bound to the Milky Way, led to the conclusion that the universe is expanding.

What is the significance of Hubble's Law in cosmology?

Hubble's Law is significant in cosmology because it provides a mathematical relationship between the recessional velocities and distances of galaxies. It states that the recession velocity of a galaxy is proportional to its distance from the observer. This law supports the idea of an expanding universe and is a key piece of evidence for the Big Bang theory.

What is the Hubble constant, and what is its role in determining the age of the universe?

The Hubble constant is a value that represents the rate at which the universe is expanding. It is approximately 70 kilometers per second per megaparsec. The Hubble constant is used in the calculation of the age of the universe by reversing the expansion rate to estimate the time since the initial expansion began, which is theorized to be the Big Bang.

How does the concept of the local group of galaxies relate to the broader understanding of the universe?

The local group of galaxies, which includes the Milky Way, provides a tangible example of how galaxies interact and are distributed in the universe. Understanding the local group helps astronomers to contextualize the broader universe, as it is a smaller-scale model of the larger cosmic structures and interactions that occur on a universal scale.

What evidence beyond Hubble's observations supports the Big Bang theory?

Beyond Hubble's observations, the Big Bang theory is supported by several key pieces of evidence, including the cosmic microwave background radiation, the abundance of light elements, and the observed large-scale structure of the universe. These pieces of evidence collectively suggest that the universe began in a hot, dense state and has been expanding and cooling ever since.

Can you explain the analogy of the expanding balloon to understand the expansion of the universe?

The expanding balloon analogy is used to visualize the concept of an expanding universe. In this analogy, the surface of the balloon represents the spacetime fabric of the universe, and the dots on the balloon represent galaxies. As the balloon inflates, the dots (galaxies) move away from each other. This demonstrates that the expansion of the universe is not due to galaxies moving through space, but rather the space itself expanding.

What is the difference between a spiral galaxy and an elliptical galaxy as described in the script?

Spiral galaxies are characterized by a central bulge surrounded by a flat, rotating disk of stars, gas, and dust, with spiral arms extending from the central region. Elliptical galaxies, on the other hand, are roughly ellipsoidal in shape and have no distinct spiral arms. They are composed mostly of older stars and have little interstellar dust and gas.

Outlines

00:00

🌌 Edwin Hubble and the Discovery of Galaxies

Professor Dave discusses the historical shift in astronomical understanding, moving from a geocentric to a heliocentric model, and then to recognizing the Milky Way as just one of many galaxies. The advent of powerful telescopes in the 20th century allowed astronomers to see distant objects as separate galaxies. Edwin Hubble played a crucial role in categorizing these galaxies and provided evidence that they are moving away from each other, a discovery made possible by studying the Doppler effect in light through emission spectra. This effect causes a shift in the frequency of light, either towards the blue end (blue shift) for objects approaching us or towards the red end (red shift) for those moving away, which Hubble used to deduce the expansion of the universe.

05:06

🌠 Hubble's Law and the Expanding Universe

The second paragraph delves into the implications of Hubble's observations, leading to the understanding that the universe is expanding. This is likened to dots on the surface of a balloon moving away from each other as the balloon inflates. The concept of the Big Bang, suggesting the universe began from a single point, is introduced as a result of this expansion. Hubble's law, which relates the recession velocity of galaxies to their distance, is explained. It uses the Hubble constant to calculate the distance of galaxies based on their redshift. The paragraph concludes by setting the stage for further discussion on the evidence supporting the Big Bang theory.

Keywords

💡Edwin Hubble

Edwin Hubble was an American astronomer who made significant contributions to the field of astronomy. He is best known for his discovery that galaxies are moving away from each other, which provided evidence for the expanding universe theory. In the video, Hubble's work is central to understanding the concept of an expanding universe and the development of the Big Bang theory.

💡Milky Way Galaxy

The Milky Way Galaxy is our home galaxy, containing our Solar System along with billions of other stars, planets, and celestial bodies. In the script, the realization that all the stars in the night sky are part of the Milky Way Galaxy was a significant step in understanding the scale of the universe.

💡Nebulae

Nebulae are vast clouds of gas and dust in space, often seen as glowing patches in long-exposure photographs of the night sky. The script explains that early astronomers thought some nebulae were part of the Milky Way, but later, with the help of powerful telescopes, they were identified as separate galaxies.

💡Galaxies

Galaxies are massive, gravitationally bound systems consisting of stars, stellar remnants, gas, dust, and dark matter. The video script discusses how Edwin Hubble categorized different types of galaxies, such as spiral and elliptical, and how his observations led to the understanding that galaxies are moving away from each other.

💡Doppler Shift

The Doppler shift is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. In the context of the video, the Doppler effect is used to explain how the redshift observed in galaxies indicates that they are moving away from us, which supports the idea of an expanding universe.

💡Redshift

Redshift refers to the shift of a spectral line towards the red end of the spectrum, indicating that an object is moving away from the observer. The video script explains that Hubble observed a redshift in the emission spectra of galaxies, which was a key piece of evidence for the expanding universe.

💡Big Bang

The Big Bang theory is the prevailing cosmological model that suggests that the universe began as a hot, dense point and has been expanding ever since. The video script connects the observations of galaxy redshifts to the Big Bang theory, suggesting that the universe had a beginning and has been expanding from that point.

💡Hubble's Law

Hubble's Law is a principle in physical cosmology that establishes a relationship between the redshift of galaxies and their distances from the observer. The video script mentions Hubble's Law as a fundamental concept that describes the observed expansion of the universe, with the Hubble constant being a key parameter in this law.

💡Hubble Constant

The Hubble Constant is a value that describes the rate at which the universe is expanding. It is used in Hubble's Law to calculate the recession velocity of galaxies. The video script discusses the Hubble Constant as a critical parameter in understanding the dynamics of the universe's expansion.

💡Cosmological Model

A cosmological model is a theoretical framework that describes the large-scale structure and evolution of the universe. The video script refers to the Big Bang as a cosmological model that explains the origin and expansion of the universe, supported by various observations, including galaxy redshifts.

💡Spacetime Fabric

The term 'spacetime fabric' is used metaphorically in the video script to describe the structure of the universe where space and time are intertwined. The analogy of a balloon expanding with dots representing galaxies helps illustrate how the fabric of spacetime stretches as the universe expands.

Highlights

Edwin Hubble's contributions to astronomy and the categorization of galaxies.

The shift from a geocentric to a heliocentric model of the universe.

The realization that stars in the night sky are suns with potentially their own planets.

The discovery that nebulae are actually separate galaxies outside the Milky Way.

The development of modern telescopes allowing for the observation of distant galaxies.

The categorization of galaxies into spiral, elliptical, and other varieties.

Hubble's development of categories and subcategories for galaxies.

Observational data leading to the interpretation of galaxies moving away from each other.

Explanation of the Doppler shift and its significance in understanding galaxy motion.

The difference between blue shift and red shift in the context of galaxy motion.

Hubble's observation of redshift in galaxies as evidence of an expanding universe.

The analogy of the expanding balloon to illustrate the expansion of the universe.

The concept that the universe must have originated from a single point in the past.

Hubble's law relating recession velocity to distance of galaxies.

The Hubble constant and its role in calculating cosmic distances.

The evidence supporting the Big Bang cosmological model.

The importance of additional evidence beyond Hubble's observations for the Big Bang theory.

Transcripts

Professor Dave here, I wanna tell you about Edwin Hubble.

Earlier in the series, we learned about the incredible history of astronomy, from ancient

times all the way up until the 20th century.

A tremendous amount of knowledge was gathered about the solar system in that time, and as

a result, the center of our universe shifted from the Earth to the Sun, with the planets

moving around in elliptical orbits.

It was then realized that all the stars in the night sky are suns just like ours, potentially

with planets of their own, and this realization caused our perceived universe to grow tremendously

in the blink of an eye, to reveal the true scope of the Milky Way Galaxy.

Then, in the early 20th century, something even more incredible happened.

Telescopes became powerful enough for us to see that many objects we had thought were

nebulae or other such objects within our galaxy, are actually not those things at all.

These are entire galaxies unto themselves, just like our own Milky Way galaxy.

All the billions of stars we see in the night sky turn out to belong to just one little

island of stars out of hundreds of billions of similar and very distant little islands,

and what we had thought was the entire universe, was revealed to be just a speck of dust in

a far grander structure.

We didn’t figure this out until the 20th century, because these galaxies are just so

far away.

We already learned about the local group that the Milky Way is a part of, and even these

closest galaxies are millions of light years away, a distance so great that the stars in

our own galaxy suddenly seem a stone’s throw away by comparison.

It took huge, powerful modern telescopes to get images with good enough resolution to

see that these were not simple structures, but rather swirling masses of many stars,

and we began to categorize them as spiral galaxies, elliptical galaxies, and all the

other varieties we learned about earlier in the series.

One person who made considerable contributions to the task of categorizing galaxies was Edwin

Hubble, and he developed the categories and subcategories we use today.

But that’s not all that Hubble did.

He gathered data that would later allow for the interpretation that the galaxies are all

moving away from one another.

This was determined by looking at emission spectra for many galaxies.

To understand how this is possible, we must learn about a phenomenon called the Doppler

shift, or the Doppler effect.

Say we have some source of waves, which could be acoustic waves, or sound, just as well

as electromagnetic waves, or light.

Say that these waves are emitted with a constant frequency.

Now let’s say that the source of these waves is in motion with respect to our frame of

reference, like a car.

When the car is coming towards us, the sound waves will be of a shorter wavelength than

if the car was stationary.

This is because the car is moving closer to us in between the emission of each wave.

If the wavelength is shorter, the frequency is greater, and we will hear the sound as

being pitched up.

Once the car passes and is moving away, the wavelength will increase, because the car

will travel some distance away from us in between the emission of each wave.

A longer wavelength means a lower frequency, and we will hear the sound as being pitched down.

Higher pitch coming towards us, lower pitch moving away from us, and that’s why a vehicle

passing by will sound like this.

Precisely the same principle works with electromagnetic waves, or light.

A source of visible light that is moving towards us will have its frequency increase, and the

wavelength will therefore decrease, towards the blue end of the visible spectrum.

We call this a blue shift.

If it is moving away from us, the frequency will decrease, and the wavelength will therefore

increase, towards the red end of the visible spectrum.

We call this a red shift.

These shifts are easy to observe on an emission spectrum, because we know where the emission

lines for hydrogen and other elements ought to be.

If an object gives emission lines that are shifted from these values, it’s a very clear

indication of the motion of that object.

When Hubble observed the emission spectra for a variety of galaxies, it became apparent

that almost every single galaxy in the observable universe is redshifted, meaning they are almost

all moving away from us, with the only exceptions being a handful of galaxies nearby that are

gravitationally bound to the Milky Way.

Rather than assuming the Milky Way is an especially repulsive galaxy that the others are fleeing

from, astronomers realized that the universe itself must be expanding.

If we imagine that the dots on this balloon represent galaxies, and the surface of the

balloon represents the spacetime fabric of the universe, as the balloon expands, all

of the dots get spread apart.

All of them are moving away from all the others, and no matter which dot you’re standing

on, all the others will appear to recede from you.

If the universe is expanding, then it must have once all been very close together.

This fact was the first piece of evidence that suggested the birth of the universe from

a single point, at some specific time in the finite past.

But as if this realization wasn’t shocking enough, Hubble also realized that the recession

velocities of the galaxies increased with greater distance.

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

This relationship is stated in Hubble’s law, a very simple equation relating recession

velocity, measured in kilometers per second, and distance, measured in megaparsecs, which

are equivalent to one million parsecs, or 3.26 million light years.

This equation also utilizes H, or the Hubble constant, equal to about 70 kilometers per

second per megaparsec, with some dispute over its precise value.

We can use this equation to find the approximate distance to any distant galaxy in megaparsecs,

given its recession velocity that we can determine from the red shift in its emission spectrum.

There is much more to discuss regarding Hubble’s law, which we will get to a bit later.

First, we want to make good on our promise from the beginning of this series.

Hubble’s realization that the galaxies are all receding from one another was the first

observation that suggested a cosmological model like the Big Bang.

But this isn’t nearly enough evidence to be conclusive.

So what else did we figure out to corroborate this model?

Let’s move forward and go over the evidence for the Big Bang.