Ask Ethan: What Causes Light To Redshift?
“If light is moving across space that’s expanding, is the speed credited with the underlying space expansion? […] A pitcher throwing a ball from a standstill throws at 100mph, but that same pitch from a platform moving at 25mph flies at 125mph. Is it like that for light? What does a red or blue shift mean in terms of the speed of light?“
The speed of light in a vacuum is always the same: 299,792,459 m/s. No matter what else happens, so long as you’re traveling through empty space, that’s the speed of your massless particle. So what happens when something boosts it or resists its motion? The light can’t gain or lose speed, so all it can do is gain or lose energy, which means the light blueshifts or redshifts, respectively. It might seem like there would be one universal cause for why light would redshift or blueshift, but it turns out that there are at least five independent factors that all come into play. Yes, it turns out that on the largest cosmic scales, the expansion of the Universe is the dominant factor, but other effects are important as well!
Come learn what really causes light to redshift in the Universe, in as comprehensive a fashion as possible.
Ask Ethan: How Can We See 46.1 Billion Light-Years Away In A 13.8 Billion Year Old Universe?
“If the limit of what we could see in a 13.8 billion year old Universe were truly 13.8 billion light-years, it would be extraordinary evidence that both General Relativity was wrong and that objects could not move from one location to a more distant location in the Universe over time. The observational evidence overwhelming indicates that objects do move, that General Relativity is correct, and that the Universe is expanding and dominated by a mix of dark matter and dark energy.
When you take the full suite of what’s known into account, we discover a Universe that began with a hot Big Bang some 13.8 billion years ago, has been expanding ever since, and whose most distant light can come to us from an object presently located 46.1 billion light-years away. The space between ourselves and the distant, unbound objects we observe continues to expand at a rate of 6.5 light-years per year at the most distant cosmic frontier. As time goes on, the distant reaches of the Universe will further recede from our grasp.”
The fabric of space is expanding, and this is perhaps the most counterintuitive thing of all. Back in the earliest stages of the Big Bang, a point in space no farther away from us than the length of a city block could have emitted a photon, and it would take that photon a remarkable 13.8 billion years just to reach us. Moreover, that photon would journey for a total of 13.8 billion light-years before arriving at our eyes, and an object located at the exact same point in space where it was initially emitted from would now be 46.1 billion light-years away from us.
Why is this? Because that’s what the laws of General Relativity, coupled with our knowledge of what’s present in the Universe, demand of space and time. Come get the full story today!
This Is How Distant Galaxies Recede Away From Us At Faster-Than-Light Speeds
“All the galaxies in the Universe beyond a certain distance appear to recede from us at speeds faster than light. Even if we emitted a photon today, at the speed of light, it will never reach any galaxies beyond that specific distance. It means any events that occur today in those galaxies will not ever be observable by us. However, it’s not because the galaxies themselves move faster than light, but rather because the fabric of space itself is expanding.
In the 7 minutes it took you to read this article, the Universe has expanded sufficiently so that another 15,000,000 stars have crossed that critical distance threshold, becoming forever unreachable. They only appear to move faster than light if we insist on a purely special relativistic explanation of redshift, a foolish path to take in an era where general relativity is well-confirmed. But it leads to an even more uncomfortable conclusion: of the 2 trillion galaxies contained within our observable Universe, only 3% of them are presently reachable, even at the speed of light.
If we care to explore the maximum amount of Universe possible, we cannot afford to delay. With each passing moment, another chance for encountering intelligent life forever slips beyond our grasp.”
If you look at a galaxy, chances are you’ll see that it appears to be receding away from us, as its light is redshifted. The more distant you look, the greater the redshift, and hence, the faster the implied recession speed. But this interpretation runs into problems very quickly: by the time you’re looking at galaxies more than 13-to-15 billion light-years away, they start to appear as though they’re receding faster than the speed of light!
Impossible, you say? Sure, if you only consider special relativity. If you insist on general relativity, it all falls into place. Here’s how.
This One Thought Experiment Shows Why Special Relativity Isn’t The Full Story
“In Einstein’s initial formulation of General Relativity way back in 1916, he mentioned the gravitational redshift (and blueshift) of light as a necessary consequence of his new theory, and the third classical test, after the precession of Mercury’s perihelion (already known at the time) and the deflection of starlight by a gravitational source (discovered during a total solar eclipse in 1919).
Although a thought experiment is an extremely powerful tool, practical experiments didn’t catch up until 1959, where the Pound-Rebka experiment finally measured a gravitational redshift/blueshift directly. Yet just by invoking the idea that energy must be conserved, and a basic understanding of particle physics and gravitational fields, we can learn that light must change its frequency in a gravitational field.”
If a photon flies through space towards Earth, it must gain energy and become bluer in nature as it approaches Earth’s surface. This idea, of a gravitational redshift or blueshift, dictates how a photon must change in energy in the presence of a gravitational field. Yet this effect, which only exists in General Relativity, could have been predicted as soon as special relativity was discovered by one simple thought experiment: to consider a particle-antiparticle pair dropped from high above the surface of the Earth, but to let the annihilation occur at varying locations.
If you considered that, you’d immediately realize how special relativity was insufficient for describing our Universe! Come learn how to reason it out for yourself today!
Ask Ethan: Could ‘Cosmic Redshift’ Be Caused By Galactic Motion, Rather Than Expanding Space?
“When we observe a distant galaxy, the light coming from the galaxy is redshifted either due to expansion of space or actually the galaxy is moving away from us. How do we differentiate between the cosmological redshift and Doppler redshift? I have searched the internet for answers but could not get any reasonable answer.”
It’s true: the farther away we look, the greater we find a galaxy’s redshift to be. But why is that? You may have heard the (correct) answer: because space is expanding. But how do we know that? Couldn’t something else be causing this redshift?
The answer is yes, there are actually four other explanations for cosmic redshift that all make sense. But the beauty of science is that there are observational tests we can perform to tell these various scenarios apart! We’ve done those tests, of course, and concluded the Universe is expanding, but wouldn’t you like to know how?
I bet you would! Come and find out how we know that cosmic redshift is caused by the expansion of the Universe, and learn where the alternatives fall apart.
Ask Ethan: If The Universe Ends In A Big Crunch, Will All Of Space Recollapse?
“When you describe the Big Crunch, you talk about a race between gravity and the expansion of space. It’s not clear to me that if gravity wins that race, whether space stops expanding, or simply that the matter in space stops expanding. I’d love to hear your explanation of this.”
The Universe is expanding, and we can confirm this by looking at the relationship between how redshifted a galaxy’s light is compared with how far away it is from us. But if these galaxies, at some point in the far future, stop being redshifted and start moving closer and closer to us again, does that necessarily mean that the fabric of space is contracting? Is all of space necessarily recollapsing? Or could the galaxies simply be moving towards us, owing to some massive attraction, while the fabric of space doesn’t recollapse at all? Does a Big Crunch necessarily equate to a recollapsing Universe?
Even though we don’t know whether dark energy will reverse itself or not, we do know the answer to this question, and yes, a Big Crunch does mean recollapse! Find out why on this edition of Ask Ethan.
This Is How We Will Discover The Most Distant Galaxy Ever
“Sometime in the distant past, likely when the Universe was less than 2% its current age, the very first galaxy of all formed when massive star clusters merged together, resulting in an unprecedented burst of star formation. The high-energy light from these stars struggles to escape, but the longer-wavelength light can penetrate farther through neutral atoms. The expansion of the Universe redshifts all the light, stretching it far beyond anything Hubble could potentially observe, but next-generation infrared telescopes should be able to catch it. And if we observe the right part of the sky, with the right instruments, for a sufficiently long time to reveal the right details about these objects, we’ll push back the cosmic frontier of the first galaxies even farther.
Somewhere, the most distant, first galaxy of all is out there, waiting to be discovered. As the 2020s approach, we can feel confident that we’ll not only shatter the current cosmic record-holder, but we know exactly how we’ll do it.”
13.8 billion years ago, our Universe as-we-know-it began with the hot Big Bang. There were no stars or galaxies back then; there weren’t even bound structures of any type. Everything was too energetic, and would immediately be destroyed by the unfathomably high temperatures and energies that every particle possessed. Yet, with time, the Universe expanded and cooled. Protons, nuclei, and neutral atoms formed; overdense regions gravitationally pulled-in mass and matter; stars were born, lived, died, and new stars were born in their aftermath. At some point, the first large star clusters merged together, passing a critical threshold and forming the first galaxy in the Universe.
That’s what we want to find. We’ve gone back to when the Universe was just 3% its present age, but that’s not enough. We must go father. We must find the first one. Here’s how we’ll do it.
Ask Ethan: If Light Contracts And Expands With Space, How Do We Detect Gravitational Waves?
“If the wavelength of light stretches and contracts with space-time, then how can LIGO detect gravitational waves. [Those waves] stretch and contract the two arms of the LIGO detector and so the the light waves within the the two arms [must] stretch and contract too. Wouldn’t the number of wavelengths of light in each arm remain the same hence cause no change in the interference pattern, rendering [gravitational waves] undetectable?”
Three years ago, we detected the very first gravitational wave ever seen, as the signal from two massive, merging black holes rippled through the Universe, carrying with it the energy of three solar masses turned into pure energy via Einstein’s E = mc^2. Since that time, we’ve discovered more gravitational waves, mostly from black hole-black hole mergers but also from a neutron star-neutron star merger.
But how did we do it? The LIGO detectors function by having two perpendicular laser beams bounce back-and-forth in a long vacuum chamber, only to recombine them at the end. As the gravitational waves pass through, the arm lengths extend and compress, changing the path length. But the wavelength of the light inside changes, too! Doesn’t this mean the effects should cancel out, and we shouldn’t see an interference pattern?
It’s what you might intuit, but it’s not right. The scientific truth is fascinating, and allows us to detect these waves anyway. Here’s how it all works!
Understanding Doppler effect using a ripple tank
Doppler effect is the increase (or decrease) in the frequency of sound, light, or
other waves as the source and observer move toward (or away from) each
There are many ways to demonstrate the Doppler effect but this one with a
ripple tank is one of the most intuitive means to clearly understand
the underlying concept.
* What is a ripple tank and what can you do with them ?
** Understanding interference using ripple tanks
When Will We Break The Record For Most Distant Galaxy Ever Discovered?
“Finally, beyond a certain distance, the Universe hasn’t formed enough stars to reionize space and make it 100% transparent.
We only perceive galaxies in a few serendipitous directions, where copious star-formation occurred.
In 2016, we fortuitously discovered GN-z11 at a redshift of 11.1: from 13.4 billion years ago.
But recent, indirect evidence suggests stars formed at even greater redshifts and earlier times.“
It was only a couple of years ago that we set the current record for where the most distant galaxy is: from 13.4 billion years ago, when the Universe was just 3% its current age. This record is unlikely to be broken with our current set of observatories, as discovering a galaxy this distant required a whole bunch of unlikely, serendipitous phenomena to line up at once. But in 2020, the James Webb Space Telescope will launch: an observatory optimized for finding exactly the kinds of galaxy that push past the limits of what Hubble can do. We fully expect to not only break the record for most distant galaxy ever discovered, but to learn, for the first time, exactly where and when the first galaxies in the Universe truly formed.
Until then, it’s lots of fun to speculate as to when and where they might be, but it will take the observations of a lifetime to smash this cosmic record!