APOD: The Coma Cluster of Galaxies (2018 Mar 26)

[neufer]

Given that the universe is expanding, has there ever been an observation of a distant galaxy that completely disappeared? Admittedly that is a very long shot but I wonder what might be learned?

You mean a galaxy that we could see, and then some time later couldn't? No. We can't see that close to the edge of the visible universe (and never will be able to do so using electromagnetic radiation).

Logically, if there are galaxies we can see and galaxies we can't, then there is a transition region. It would seem like that region is like an "event horizon". And it's moving faster than the speed of light which may be why you phrased your answer that way. I have found some discussion on this in "Ask an astronomer", to wit: "As a consequence of their great speeds, these galaxies will likely not be visible to us forever; some of them are right now emitting their last bit of light that will ever be able to make it all the way across space and reach us (billions of years from now). After that, we will observe them to freeze and fade, never to be seen again". However that sentence refers to present time, i.e. "right now" whatever that is, whereas if the universe is old enough then there should be some galaxies well in the past whose light is their last light to us. That would seem to be right up to the edge but maybe the universe isn't old enough yet? That discussion was here. http://curious.astro.cornell.edu/about- ... termediate The penultimate paragraph seems to suggest there are some galaxies ready to blink off.

• Distant galaxies never die,
  Never die, never die,
  Distant galaxies never die,
  They simply fade away.

Will the cosmic microwave background radiation eventually disappear?

Frank Heile, Ph.D. in Physics from Stanford University
Updated Oct 1, 2017

<<About the CMB now:

The Cosmic Microwave Background radiation (CMB) is radiation that was emitted when the universe was about 380,000 years old. At the time it was emitted the radiation was black body radiation at a temperature of about 3000 K. Today that same radiation has a black body temperature of about 2.725 K. The ratio of those two temperatures is about 1100 and this is, in fact, the factor by which the linear size of the universe has expanded between 380,000 years after the big bang to now (13.8 billion years after the big bang). So, since this expansion will continue into the future, the temperature of the CMB will continue to fall - asymptotically approaching absolute zero - but never quite reaching 0 K. So, let's call this decrease in temperature a form of “dissipation” of the CMB.

How the CMB will change in the future:

We are entering an era where the universe will accelerate the expansion rate. To be specific, the universe will double in size every 8 billion
years (or so) from now on. This means that the z factor of 1100 mentioned earlier will double every 8 billion years. Now a doubling of z
will mean that the energy of each photon will decrease by a factor of 2 and the energy density of the CMB will decrease by a factor of 8 every 8 billion years! So let’s figure out when the wavelength of the CMB photons will begin to exceed 1 light−year. This is significant because you need an antenna to be approximately the size of the wavelength (or larger) to detect the energy of a single photon of radiation. Effectively, the CMB radiation will be undetectable if its wavelength is 1 light−year. Right now the wavelength of the CMB radiation is about 1 mm. So, in “just” about 500 billion years the CMB will have a wavelength of 1 light year and will be essentially undetectable.

So, although the CMB will never disappear from a theoretical point of view, from a practical point of view the CMB will be completely undetectable 500 billion years from now.>>

They hunted till darkness came on, but they found
Not a button, or feather, or mark,
By which they could tell that they stood on the ground
Where the Baker had met with the Snark.

In the midst of the word he was trying to say,
In the midst of his laughter and glee,
He had softly and suddenly vanished away—
For the Snark was a Boojum, you see.

[Ann]

Given that the universe is expanding, has there ever been an observation of a distant galaxy that completely disappeared? Admittedly that is a very long shot but I wonder what might be learned?

You mean a galaxy that we could see, and then some time later couldn't? No. We can't see that close to the edge of the visible universe (and never will be able to do so using electromagnetic radiation).

Logically, if there are galaxies we can see and galaxies we can't, then there is a transition region. It would seem like that region is like an "event horizon". And it's moving faster than the speed of light which may be why you phrased your answer that way. I have found some discussion on this in "Ask an astronomer", to wit: "As a consequence of their great speeds, these galaxies will likely not be visible to us forever; some of them are right now emitting their last bit of light that will ever be able to make it all the way across space and reach us (billions of years from now). After that, we will observe them to freeze and fade, never to be seen again". However that sentence refers to present time, i.e. "right now" whatever that is, whereas if the universe is old enough then there should be some galaxies well in the past whose light is their last light to us. That would seem to be right up to the edge but maybe the universe isn't old enough yet? That discussion was here. http://curious.astro.cornell.edu/about- ... termediate The penultimate paragraph seems to suggest there are some galaxies ready to blink off.

I trust Chris' understanding of telescopes and other detectors and our ability to make observations. However, I do think you are right when you say that there should be a transition region.

But think of it like this. Galaxies that are very close to "blinking out" due to the expansion of the Universe must be exceedingly faint. Even if we can, theoretically, detect these galaxies, they are so faint that they must be enormously hard to find. For practical purposes, they may already be "invisible".

Ann

[neufer]

I do think you are right when you say that there should be a transition region. But think of it like this. Galaxies that are very close to "blinking out" due to the expansion of the Universe must be exceedingly faint. Even if we can, theoretically, detect these galaxies, they are so faint that they must be enormously hard to find. For practical purposes, they may already be "invisible".

There are NO galaxies in the visible universe that are at all close to "blinking out" due to the expansion of the Universe!

Even the Cosmic Background Radiation (far far behind all the galaxies in the visible universe)
is not close to "blinking out" due to the expansion of the Universe!

The CBR and the galaxies in the visible universe (not in our Local Group) are simply expanding away at a slow
but exponential rate such that they will only be visible with infrared or radio telescopes in the distant future.

Everything today in the distant visible universe will still be visible 115 million years from now
... just at a wavelength ~ 1% longer than it is today.

It is highly unlikely, however, that there will be any Homo Sapiens around to observe this "altered" universe.

[Ann]

I do think you are right when you say that there should be a transition region. But think of it like this. Galaxies that are very close to "blinking out" due to the expansion of the Universe must be exceedingly faint. Even if we can, theoretically, detect these galaxies, they are so faint that they must be enormously hard to find. For practical purposes, they may already be "invisible".

There are NO galaxies in the visible universe that are at all close to "blinking out" due to the expansion of the Universe!

Even the Cosmic Background Radiation (far far behind all the galaxies in the visible universe)
is not close to "blinking out" due to the expansion of the Universe!

The CBR and the galaxies in the visible universe (not in our Local Group) are simply expanding away at a slow
but exponential rate such that they will only be visible with infrared or radio telescopes in the distant future.

Everything today in the distant visible universe will still be visible 115 million years from now
... just at a wavelength ~ 1% longer than it is today.

It is highly unlikely, however, that there will be any Homo Sapiens around to observe this "altered" universe.

Okay, Art! Thanks for enlightening me (and other people here)!

Ann

[alter-ego]

[/quote]

• Distant galaxies never die,
  Never die, never die,
  Distant galaxies never die,
  They simply fade away.

There are NO galaxies in the visible universe that are at all close to "blinking out" due to the expansion of the Universe!

Even the Cosmic Background Radiation (far far behind all the galaxies in the visible universe)
is not close to "blinking out" due to the expansion of the Universe!

The CBR and the galaxies in the visible universe (not in our Local Group) are simply expanding away at a slow
but exponential rate such that they will only be visible with infrared or radio telescopes in the distant future.

This is all true, and yet "blinking out" does occur in our accelerating flat universe, just not in the sense of vanishing from view as so far discussed. Given the present cosmological parameters, we currently have a comoving event horizon of ~16Gly (z ~1.75). This doesn't mean we can't see objects further than that. We can, theoretically all the way out to the CMB. Instead it means that light emitted today will never reach us. For z ≥ 1.75, we will NEVER see photons emitted later than the present age of the universe (~13.7Gyr). From now through the infinite future, an object with redshift of 1.75, and distance, will continue to increase (get redder and fainter without limit), but the emission time will only increase from 3.7Gyr we observe today to a limit of 13.7Gyr, i.e. over an infinite observation time span, we'll witness only 10Gyr of red-shifted time span. Ironically, as slow as we think cosmological time is, cosmological time dilation leads to cosmological evolution observed in slow motion.

[Ann]

• Distant galaxies never die,
  Never die, never die,
  Distant galaxies never die,
  They simply fade away.

There are NO galaxies in the visible universe that are at all close to "blinking out" due to the expansion of the Universe!

Even the Cosmic Background Radiation (far far behind all the galaxies in the visible universe)
is not close to "blinking out" due to the expansion of the Universe!

The CBR and the galaxies in the visible universe (not in our Local Group) are simply expanding away at a slow
but exponential rate such that they will only be visible with infrared or radio telescopes in the distant future.

This is all true, and yet "blinking out" does occur in our accelerating flat universe, just not in the sense of vanishing from view as so far discussed. Given the present cosmological parameters, we currently have a comoving event horizon of ~16Gly (z ~1.75). This doesn't mean we can't see objects further than that. We can, theoretically all the way out to the CMB. Instead it means that light emitted today will never reach us. For z ≥ 1.75, we will NEVER see photons emitted later than the present age of the universe (~13.7Gyr). From now through the infinite future, an object with redshift of 1.75, and distance, will continue to increase (get redder and fainter without limit), but the emission time will only increase from 3.7Gyr we observe today to a limit of 13.7Gyr, i.e. over an infinite observation time span, we'll witness only 10Gyr of red-shifted time span. Ironically, as slow as we think cosmological time is, cosmological time dilation leads to cosmological evolution observed in slow motion.

Would you clarify that, please, alter-ego?

Ann

[Curiouser A. Curiouser]

Whatever the exact processes, it does seem that galaxies that have long been members of massive galaxy clusters lose their ability to form new stars, and become shapeless and yellow.

Do you suppose that as the universe expands the ISM will become cooler and less dense thus allowing these giant ellipticals to restart star formation?

The Universe doesn't expand within galaxy clusters, as gravity holds everything together.

Please forgive me if this is an entirely naive or ignorant question but, does the density of the universe remain the same locally as spacetime expands all around?

[neufer]

• Distant galaxies never die,
  Never die, never die,
  Distant galaxies never die,
  They simply fade away.

There are NO galaxies in the visible universe that are at all close to "blinking out" due to the expansion of the Universe!

Even the Cosmic Background Radiation (far far behind all the galaxies in the visible universe)
is not close to "blinking out" due to the expansion of the Universe!

The CBR and the galaxies in the visible universe (not in our Local Group) are simply expanding away at a slow
but exponential rate such that they will only be visible with infrared or radio telescopes in the distant future.

This is all true, and yet "blinking out" does occur in our accelerating flat universe, just not in the sense of vanishing from view as so far discussed. Given the present cosmological parameters, we currently have a comoving event horizon of ~16Gly (z ~1.75). This doesn't mean we can't see objects further than that. We can, theoretically all the way out to the CMB. Instead it means that light emitted today will never reach us. For z ≥ 1.75, we will NEVER see photons emitted later than the present age of the universe (~13.7Gyr). From now through the infinite future, an object with redshift of 1.75, and distance, will continue to increase (get redder and fainter without limit), but the emission time will only increase from 3.7Gyr we observe today to a limit of 13.7Gyr, i.e. over an infinite observation time span, we'll witness only 10Gyr of red-shifted time span. Ironically, as slow as we think cosmological time is, cosmological time dilation leads to cosmological evolution observed in slow motion.

Would you clarify that, please, alter-ego?

The classic Achilles and the tortoise race paradox

---

Consider the classic Achilles and the tortoise race paradox where:

1) Achilles represents a recent photon from a distant galaxy and

2) The (Frisbee shaped) Earth is sitting on the back of a tortoise... that only "moves" to the right because the race course, itself, is stretching.

In the diagram to the left the race course stretches at constant rate such that the Achilles photon catches up with the tortoise Earth in a finite amount of time (although with an infinite number of hops).

However, if the race course stretches at an exponential rate then the 2nd diagram recycles back up to the top diagram and the Achilles photon never catches up with the tortoise Earth.

[Ann]

The classic Achilles and the tortoise race paradox

---

Consider the classic Achilles and the tortoise race paradox where:

1) Achilles represents a recent photon from a distant galaxy and

2) The (Frisbee shaped) Earth is sitting on the back of a tortoise... that only "moves" to the right because the race course, itself, is stretching.

In the diagram to the left the race course stretches at constant rate such that the Achilles photon catches up with the tortoise Earth in a finite amount of time (although with an infinite number of hops).

However, if the race course stretches at an exponential rate then the 2nd diagram recycles back up to the top diagram and the Achilles photon never catches up with the tortoise Earth.

Click to view full size image

Painting by Salvador Dalí.

Thanks, Art! Yes, I remember the tortoise and Achilles (although when I was told about the parable, the tortoise was racing against a hare).

Okay! So if the (frisbee shaped ) Earth is sitting on the back of the tortoise, and the Universe is expanding at an exponential rate, then Achilles (or the hare) never catches up.

And there are probably elephants all the way down, too! (They must get spaghettified in an exponentially expanding Universe, as in this Salvador Dalí painting!)

Ann

[neufer]

Thanks, Art! Yes, I remember the tortoise and Achilles (although when I was told about the parable, the tortoise was racing against a hare).

I'm SHOCKED...shocked I tell you that you would mix metaphors

The moral of "The Tortoise and the Hare" has more to do with Chris Peter(rabbit)son rushing in to answer a Asterisk* question off the top of his head while I hang back Googling stuff and rechecking calculations until I conclude that Chris just might have made an error (...or not).

A 19th-century illustration of La Fontaine's Fables by Jean Grandville

---

<<"The Tortoise and the Hare" is one of Aesop's Fables and is itself a variant of a common folktale theme in which ingenuity and trickery (rather than doggedness) are employed to overcome a stronger opponent. The story concerns a Hare who ridicules a slow-moving Tortoise. Tired of the Hare's arrogant behavior, the Tortoise challenges him to a race. The hare soon leaves the tortoise behind and, confident of winning, takes a nap midway through the race. When the Hare awakes however, he finds that his competitor, crawling slowly but steadily, has arrived before him.

In Classical times the story was annexed to a philosophical problem by Zeno of Elea in one of many demonstrations that movement is impossible to define satisfactorily. The second of Zeno's paradoxes is that of Achilles and the Tortoise, in which the hero gives the Tortoise a head start in a race. The argument attempts to show that even though Achilles runs faster than the Tortoise, he will never catch up with her because, when Achilles reaches the point at which the Tortoise started, the Tortoise has advanced some distance beyond; when Achilles arrives at the point where the Tortoise was when Achilles arrived at the point where the Tortoise started, the Tortoise has again moved forward. Hence Achilles can never catch the Tortoise, no matter how fast he runs, since the Tortoise will always be moving ahead.>>

<<Zeno's paradoxes are a set of philosophical problems generally thought to have been devised by Greek philosopher Zeno of Elea (ca. 490–430 BC) to support Parmenides' doctrine that contrary to the evidence of one's senses, the belief in plurality and change is mistaken, and in particular that motion is nothing but an illusion. It is usually assumed, based on Plato's Parmenides (128a–d), that Zeno took on the project of creating these paradoxes because other philosophers had created paradoxes against Parmenides' view. Thus Plato has Zeno say the purpose of the paradoxes "is to show that their hypothesis that existences are many, if properly followed up, leads to still more absurd results than the hypothesis that they are one." Plato has Socrates claim that Zeno and Parmenides were essentially arguing exactly the same point.>>

[alter-ego]

...
Please forgive me if this is an entirely naive or ignorant question but, does the density of the universe remain the same locally as spacetime expands all around?

Well it will depend on the local mass density, and the detail cosmological expansion within that region, i.e. does it drop to zero in that higher density region or is there a small, but not zero, contribution to expansion within that region. Peculiar motion of matter will also affect the average mass density over time. Over the volume of interest, the vacuum energy density will remain constant. Therefore the mass density will determine the degree of expansion.

• For our Milky Way, assuming 6x10¹¹M☉ is contained within a 100,000 ly diameter sphere, the average mass density ≈ 3x10^-24 gm/cm³. This is about 100,000 times the average mass density of the universe, so there's no cosmological expansion taking place in our (or any) galaxy.

• Considering M31 and our Milky Way, the collective total mass ≈ 5x10¹²M☉ and contained within a 2.5x10⁶ly sphere. The average mass density ≈ 1.5x10^-27gm/cm³ which is about 45 times the average mass density of the universe. It's not clear to me that expansion is zero for this case, but it's still negligibly small if not zero.

The present consensus is that groups and clusters are dense enough to halt expansion and remain bound. Galaxies more spread out within filaments are subject to cosmological expansion. For bound systems the mass density will remain the same and the answer to your question is yes.

[C. A. Curious]

...
Please forgive me if this is an entirely naive or ignorant question but, does the density of the universe remain the same locally as spacetime expands all around?

...
The present consensus is that groups and clusters are dense enough to halt expansion and remain bound. Galaxies more spread out within filaments are subject to cosmological expansion. For bound systems the mass density will remain the same and the answer to your question is yes.

Wow that is rather startling. Locally we can expect the big crunch but the universe as a whole ends up getting what?, the big rip or maybe just very, very spartan.

[neufer]

...
Please forgive me if this is an entirely naive or ignorant question but, does the density of the universe remain the same locally as spacetime expands all around?

The present consensus is that groups and clusters are dense enough to halt expansion and remain bound. Galaxies more spread out within filaments are subject to cosmological expansion. For bound systems the mass density will remain the same and the answer to your question is yes.

Wow that is rather startling. Locally we can expect the big crunch but the universe as a whole ends up getting what?, the big rip or maybe just very, very spartan.

<<The Big Freeze is a scenario under which continued expansion results in a universe that asymptotically approaches absolute zero temperature. This scenario, in combination with the Big Rip scenario, is currently gaining ground as the most important hypothesis. In this scenario, stars are expected to form normally for 1–100 trillion years, but eventually the supply of gas needed for star formation will be exhausted. As existing stars run out of fuel and cease to shine, the universe will slowly and inexorably grow darker. Eventually black holes will dominate the universe, which themselves will disappear over time as they emit Hawking radiation. Over infinite time, there would be a spontaneous entropy decrease by the Poincaré recurrence theorem, thermal fluctuations, and the fluctuation theorem.

A related scenario is heat death, which states that the universe goes to a state of maximum entropy in which everything is evenly distributed and there are no gradients—which are needed to sustain information processing, one form of which is life. The heat death scenario is compatible with any of the three spatial models, but requires that the universe reach an eventual temperature minimum.>>