ClassX Video Lessons Youtube Channel Domain of Science 97% of Galaxies Are Moving Faster Than Light, HOW IS THIS POSSIBLE?
Have you ever wondered how galaxies can move faster than the speed of light? It sounds like a contradiction to one of the fundamental laws of physics, which states that nothing can travel faster than light. However, this intriguing phenomenon can be explained through Einstein’s theory of special relativity. The key lies in understanding that these galaxies aren’t moving through space at such speeds; instead, the space between us and them is expanding.
To grasp this concept, let’s use a couple of analogies. Imagine you’re at a birthday party for anteaters, and you’re inflating a balloon covered in ants. As the balloon expands, the ants move further apart, even though they’re not moving themselves. Here, the ants represent galaxies, and the balloon represents the expanding universe.
Now, consider baking bread with ants scattered throughout the dough. As the dough rises, the ants move apart in three-dimensional space, yet they remain the same size. This is similar to galaxies, which maintain their size due to their gravitational forces, despite the expansion of space driven by dark energy.
It’s important to note that space isn’t expanding into anything else; it’s simply expanding. As a result, galaxies that are further apart move away from each other faster. This is a natural consequence of expanding spacetime.
For example, imagine two galaxies one megaparsec apart (about 3.26 million light-years). Space is expanding at approximately 70 kilometers per second per megaparsec. Thus, these galaxies are moving away from us at 70 kilometers per second due to this expansion. A galaxy twice as far would move away twice as fast, and so on, until distant galaxies appear to move faster than light.
When we observe distant galaxies, we see them as they were in the past because the light has traveled for billions of years to reach us. For instance, the most distant galaxy we’ve observed, GN-z11, is currently 32 billion light-years away. However, we see it as it was just 400 million years after the Big Bang, about 13.5 billion years ago.
As we look further back in time, we approach the cosmic horizon, marked by the Cosmic Microwave Background and the Big Bang. Although the universe is 13.8 billion years old, the cosmic horizon is 45 billion light-years away due to cosmic expansion. Thus, we observe galaxies as they were long ago, not as they are now.
At the time of the Big Bang, everything in the observable universe was packed into a tiny volume. Over the next 400 million years, space expanded, stars formed and exploded, and gravity shaped the remnants into new stars. The light from GN-z11 traveled for 13.5 billion years, stretching as space expanded, a phenomenon known as redshift. Today, this light reaches our telescopes, revealing GN-z11’s current distance of 32 billion light-years.
Interestingly, GN-z11 likely no longer exists in its original form; its material has transformed into other stars or galaxies. Because it’s so far away, it’s moving faster than light, meaning no light it emits now will ever reach us.
Remarkably, 97% of stars and galaxies are moving away from us faster than light, creating an invisible sphere of inaccessibility about 13 billion light-years away. Even if we could travel at light speed, we could never reach beyond this boundary.
As the universe continues to expand, the sphere of inaccessibility shrinks, meaning fewer parts of the universe will ever see signals we send today. Meanwhile, the edge of the observable universe grows, allowing us to see more galaxies over time. Although we’ll see more of the universe, we’ll be in touch with less of it.
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Gather a balloon and a marker. Inflate the balloon slightly and draw several dots on its surface to represent galaxies. Continue inflating the balloon and observe how the dots move apart. This activity will help you visualize the concept of expanding space and how galaxies appear to move away from each other.
Use dough and small beads to simulate galaxies. As the dough rises, notice how the beads move apart while maintaining their size. This hands-on activity will reinforce the idea of galaxies maintaining their size due to gravitational forces, even as space expands.
Research the concept of redshift and create a presentation explaining how light from distant galaxies stretches as space expands. Include examples of how astronomers use redshift to determine the distance and speed of galaxies. This will deepen your understanding of how we observe the universe.
Create a timeline of the universe from the Big Bang to the present day. Include key events such as the formation of the first stars and galaxies, and the observation of GN-z11. This will help you contextualize the expansion of the universe and the history of cosmic events.
Organize a debate on the implications of the sphere of inaccessibility. Discuss how the expansion of the universe affects our ability to explore and communicate with distant galaxies. This activity will encourage critical thinking and exploration of the universe’s future.
Sure! Here’s a sanitized version of the transcript:
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97% of the galaxies in the observable universe are moving away from us faster than the speed of light. But how can this be true when one of the fundamental laws of the universe states that nothing can travel faster than light? The answer lies in Einstein’s theory of special relativity. The rules that dictate this only apply to objects traveling through space. These distant galaxies aren’t moving away from us because they’re moving through space, but because the space between us is expanding. Thus, the distance between us and them is increasing faster than light could travel across that space.
To help understand this unusual situation, here are a couple of popular analogies: Imagine I’m having a birthday party for anteaters and I’m blowing up a balloon covered in ants. As the balloon inflates, the ants get further away from each other, even if they’re standing still. The ants represent the galaxies, and the expanding balloon represents the expanding space.
Now, let’s improve this analogy. Imagine I’m baking some bread with ants in it for this unique anteater party. The ants are scattered throughout the dough, and as the dough rises, the ants get further away from each other again, but this time in three-dimensional space. Note that the ants don’t get any bigger; they stay the same size. This is similar to galaxies, whose gravity is much stronger than the force causing space to expand, which we refer to as dark energy. Therefore, galaxies, solar systems, stars, and planets all maintain their size despite the expanding space they inhabit.
Another important point is that space isn’t expanding into anything else; space is everything, and it’s all expanding. A key result of this is that galaxies that are further away from each other will move away from each other faster. This is a natural consequence of the expanding spacetime.
To illustrate this, let’s look at a simple one-dimensional example. Imagine two galaxies that are exactly one megaparsec away in each direction. A megaparsec is about 3.26 million light-years. According to astrophysicists, space is expanding at roughly 70 kilometers per second per megaparsec. So, these galaxies are moving away from us at 70 kilometers per second due to the expansion of space.
From the perspective of the leftmost galaxy, it would see our original galaxy moving away at 70 kilometers per second, but the second galaxy would be moving away twice as fast. Another galaxy that’s three megaparsecs away would be retreating three times as fast. As you go further away, the speed of retreat increases until you reach a point where distant galaxies are moving away from us faster than light.
This leads to some fascinating facts. When we observe a distant galaxy, the light has been traveling through space for a long time, so we see the galaxy as it was in the distant past. However, that same light tells us how far away the galaxy is right now, not how far away it was when the light was emitted. For example, let’s consider the most distant galaxy we’ve ever observed, GN-z11. It’s currently 32 billion light-years away, but we see it as it was just 400 million years after the Big Bang, some 13.5 billion years ago.
If we continue looking beyond GN-z11, we can look further back in time until we reach the limits of the cosmic horizon, which is the Cosmic Microwave Background and the Big Bang. Although the universe is 13.8 billion years old, the cosmic horizon is further away at 45 billion light-years due to cosmic expansion. So today, we observe galaxies where they are now, but as they were millions or billions of years ago.
To make this concrete, let’s tell the story of us in this galaxy at the time of the Big Bang. The atoms that will eventually make us and GN-z11 were right next to each other because everything in the observable universe was packed into a tiny volume. Over the next 400 million years, space expanded, the first generation of stars formed and exploded, and gravity pulled their remnants into the second generation of stars. This is where our atoms were 400 million years after the Big Bang when the young galaxy GN-z11, sitting just 2.6 billion light-years away, emitted some light in our direction.
This light traveled through space towards us for the next 13.5 billion years. The second generation of stars exploded, and their remnants went on to form our galaxy and our Sun, which is a third-generation star. All this time, the light from GN-z11 was being stretched as the space it moved through expanded, causing its wavelength to get longer, a phenomenon known as redshift. Today, the light finally reaches the lenses of our telescopes, and from the redshift, we can determine that GN-z11 is currently 32 billion light-years away.
However, GN-z11 doesn’t exist anymore; its material has likely turned into other stars or galaxies a long time ago. Because it’s so far away, it’s moving away from us faster than the speed of light, meaning no light it emits now will ever reach us.
So, how much of the universe is moving away from us faster than the speed of light? It turns out that 97% of the stars and galaxies are. This creates an invisible sphere of inaccessibility at about 13 billion light-years, beyond which even if you had a spacecraft that could travel at the speed of light, we would never be able to reach.
Now, let’s address the physics puzzle I mentioned at the beginning of the video: Can you calculate how many of the galaxies in the observable universe are moving away from us faster than the speed of light? All the information you need to solve this problem is contained within this video, so give it a try! I have 97% and will be posting my solution in two days. If you want to discuss your own solutions, we can use the comment section below.
Now, onto the last part of this video, which is quite mind-bending. We have two boundaries in our universe: one at the edge of the observable universe and the other at the sphere of inaccessibility, beyond which no signal we send today will ever reach. The question is: How will these boundaries move in the future? Will they get bigger, smaller, or stay the same size?
Pause the video now if you want to take a guess. Unsurprisingly, due to the accelerating expansion of space, the sphere of inaccessibility is shrinking. As time goes on, less and less of the universe will ever see any signal we emit today. In fact, every year about 20 million stars slip forever beyond our reach.
As for the edge of the observable universe, the opposite is happening; it’s increasing in size over time. This surprised me when I learned it because it seems counterintuitive regarding the expanding universe. As time passes, more and more galaxies will slip into view at the edge of the observable universe. Right now, we can see more of the universe than ever before in the history of Earth. The reason for this is that the light from galaxies currently outside the observable universe is already on its way to us, and over time, more of that light will reach us.
In the future, we will have more time to see more of the universe, but remember, we will only ever see these galaxies as they were shortly after the Big Bang. We’ll never see them as they are today. It’s a bittersweet thought that in the far future, we’ll be able to see more of the universe, but we’ll be in touch with less and less of it.
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This version maintains the core ideas while ensuring clarity and coherence.
Galaxies – Large systems of stars, interstellar gas, dust, and dark matter, bound together by gravity. – The Milky Way and Andromeda are two of the most well-known galaxies in our local group.
Light – Electromagnetic radiation that is visible to the human eye and is responsible for the sense of sight. – The speed of light in a vacuum is approximately 299,792 kilometers per second, a fundamental constant in physics.
Space – The vast, seemingly infinite expanse that exists beyond the Earth’s atmosphere, where all celestial bodies reside. – The study of space has led to the discovery of numerous exoplanets orbiting distant stars.
Expansion – The increase in distance between any two given gravitationally unbound parts of the observable universe over time. – The expansion of the universe is evidenced by the redshift of light from distant galaxies.
Universe – The totality of known or supposed objects and phenomena throughout space; the cosmos; everything that exists. – The Big Bang theory describes the origin of the universe as an expansion from a singularity.
Gravity – The natural force of attraction exerted by a celestial body, such as Earth, upon objects at or near its surface, tending to draw them toward the center of the body. – Gravity is the force that keeps planets in orbit around stars and governs the motion of galaxies.
Stars – Luminous celestial bodies made of plasma, held together by gravity, and generating energy through nuclear fusion. – The lifecycle of stars includes stages such as the main sequence, red giant, and supernova.
Redshift – The phenomenon where light or other electromagnetic radiation from an object is increased in wavelength, or shifted to the red end of the spectrum. – Redshift is a key indicator used by astronomers to determine the speed at which a galaxy is moving away from Earth.
Dark Energy – A mysterious form of energy that is hypothesized to permeate all of space and is responsible for the accelerated expansion of the universe. – Dark energy constitutes about 68% of the total energy content of the universe, according to current cosmological models.
Cosmology – The scientific study of the large scale properties of the universe as a whole. – Cosmology seeks to understand the origin, evolution, and eventual fate of the universe.
