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!
The Doppler effect and redshift of galaxies
If a stationary source produces a wave then the observed frequency will be the same regardless of where the observer is. However if the source is moving, the observed frequency will depend on whether it is moving towards or away from you. If it is moving towards you, the observed waves will be compressed and it will appear to be a higher frequency. If the source is moving away from you then the observed frequency will be elongated and appear to be a lower than the source. This is the Doppler effect.
We know our sun contains helium because of the black lines on the spectrum where it has absorbed light. Hence the absorption spectrum of helium. So when we look at a different star we see the same absorption spectrum except the position of the lines have changed because their wavelength increased and frequency decreased causing them to shift to the red end of the spectrum (hence redshift). What astronomers are finding is that the further a star is from us, the more its light is red-shifted. This tells us that distant galaxies are moving away from us and the further it is the faster it’s moving which is strong evidence for an expanding universe.
5 Questions You Were Too Embarrassed To Ask About The Expanding Universe
“5.) Are there galaxies moving away faster than the speed of light, and isn’t that forbidden? From our point of view, the space in between us and any distant point is expanding. The farther away something is, the faster it appears to recede from us. Even if the expansion rate were tiny, an object far enough away would eventually cross that threshold of any finite speed, since an expansion rate (a speed-per-distance) multiplied by a great enough distance will give you a speed as fast as you want. But this is okay in General Relativity! The law that nothing can travel faster than the speed of light only applies to an object’s motion through space, not to the expansion of space itself. In reality, the galaxies themselves only move around at speeds that are hundreds or thousands of km/s, much lower than the 300,000 km/s speed limit set by the speed of light. It’s the expansion of the Universe that causes this recession and the redshift, not a true galactic motion.”
The idea that the spatial fabric of the Universe itself is expanding, and that’s what’s behind the observed relationship between redshift and distance has long been controversial, and also long-misunderstood. After all, if more distant objects appear to recede more quickly, couldn’t there be a different explanation, like an explosion that flung many things outward? As it turns out, this isn’t a mere difference in interpretation, there are observations we can make that tell us the answer! The Universe is not expanding ‘into’ anything, despite what your intuition might tell you. The Hubble ‘constant’ isn’t actually a constant, but is rather decreasing as time goes on. The Universe looks like it’s going to expand forever, but even that scientific conclusion is subject to revision depending on what data shows in the future. And although 97% of the galaxies in the Universe are already unreachable, it isn’t a violation of relativity or a faster-than-light phenomenon that’s to blame.
Come learn the answers to five questions about the expanding Universe that many are too embarrassed to ask!