APOD: A Flight through the Hubble Ultra... (2020 Jul 26)

[sillyworm 2]

One gigantic tease.We may never reach the nearest star! What a majestic twisted trick we have been played.

[neufer]

Ahead! Warp factor 9 gazillion, Mr. Sulu!

• The warp factor : 250,000

I think that's too fast. Warp 250000 would be 15.625e15 (15.625 quadrillion) times the speed light. That's a light year in about 2.02 nanoseconds, or a Gly in 2 seconds, or 27 secs to cover 13.8 Gly. Since the length of the video is 58 seconds, we'd only need to go about half the effective speed of warp 250000, which, by the magic of cubes is "only" about warp 198000

The z=8 galaxies have a comoving radial distance of ~30 Gly:
http://www.astro.ucla.edu/%7Ewright/CosmoCalc.html

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<<The observable universe is a spherical region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time. According to calculations, the current comoving—proper distance, which takes into account that the universe has expanded since the light was emitted—to particles from which the cosmic microwave background radiation (CMBR) was emitted, which represents the radius of the visible universe, is about 45.7 billion light-years, while the comoving distance to the edge of the observable universe is about 46.6 billion light-years, about 2% larger. The radius of the observable universe is therefore estimated to be about 46.5 billion light-years.

As the universe's expansion is accelerating, all currently observable objects will eventually appear to freeze in time, while emitting progressively redder and fainter light. For instance, objects with the current redshift z from 5 to 10 will remain observable for no more than 4–6 billion years. In addition, light emitted by objects currently situated beyond a certain comoving distance (currently about 19 billion parsecs) will never reach Earth.>>

[neufer]

Click to play embedded YouTube video.

[alter-ego]

...
As the universe's expansion is accelerating, all currently observable objects will eventually appear to freeze in time, while emitting progressively redder and fainter light. For instance, objects with the current redshift z from 5 to 10 will remain observable for no more than 4–6 billion years. In addition, light emitted by objects currently situated beyond a certain comoving distance (currently about 19 billion parsecs) will never reach
Earth.^[17]

Poorly written at best, just plain wrong at worst.

For instance, objects with the current redshift z from 5 to 10 will remain observable for no more than 4–6 billion years.

• Per the referenced paper: Objects with the current redshift z from 5 to 10 "will be only visible up to an age of 4-6 billion years" (entirely different meaning). It takes an infinite amount of time to reach the observable limiting age of these objects. After 45Gyr to 50Gyr from now the observed ages will be within 5% of their asymptote limits.
  → For present ΛCDM expansion parameters, these objects will theoretically be visible forever, although progressively getting fainter and redshifted. Presently, the objects' ages range from 0.5 - 1Gy. It's ironic that as slow as cosmic evolution is, one could say these objects will evolve in slow motion until apparently nearly frozen in time 50Gyr from now.

In addition, light emitted by objects currently situated beyond a certain comoving distance (currently about 19 billion parsecs) will never reach Earth.

• This statement is not from the paper and looks plain wrong. What comoving distance (i.e. event horizon or particle horizon) is being referred to? Without more context, 19Gpc makes no sense - it is ~36% larger than the observable universe (aka particle horizon). Correctly stated in Wiki's first paragraph (quoted by Art), the particle horizon currently has a fundamental, limiting radius ~47Gly ( or ~14Gpc).
  → Per the current ΛCDM model, NOTHING beyond this radius has been or will ever be observable. It's a hard-wall boundary to anything observable beyond it. Of course, as time progresses, the particle horizon grows as space expands without new energy/objects added to the system.

• Regarding the Even Horizon, for any object with z ≥ ~1.8, the light emitted today will never reach the Earth. Those higher-z objects are beyond our Event Horizon (comoving distance ≈ 16Gly) - Also not 19Gpc.

[neufer]

...In addition, light emitted by objects currently situated beyond a certain comoving distance (currently about 19 billion parsecs) will never reach Earth.

• This statement is not from the paper and looks plain wrong. What comoving distance (i.e. event horizon or particle horizon) is being referred to? Without more context, 19Gpc makes no sense - it is ~36% larger than the observable universe (aka particle horizon). Correctly stated in Wiki's first paragraph (quoted by Art), the particle horizon currently has a fundamental, limiting radius ~47Gly ( or ~14Gpc).
  → Per the current ΛCDM model, NOTHING beyond this radius has been or will ever be observable. It's a hard-wall boundary to anything observable beyond it. Of course, as time progresses, the particle horizon grows as space expands without new energy/objects added to the system.

• Regarding the Even[t] Horizon, for any object with z ≥ ~1.8, the light emitted today will never reach the Earth. Those higher-z objects are beyond our Event Horizon (comoving distance ≈ 16Gly) - Also not 19Gpc.

Granted...however, it is still not clear (to me at least) if the 2001 paper [cue the _Also sprach Zarathustra_ music] takes into account the fact that the Hubble [space] constant is decreasing in time such that ~14Gpc may not in the future represent the distance at which objects recede at the speed of light.

Is the universe expanding faster than the speed of light?
Answered by Dave Rothstein
Last updated February 10, 2016.

<<Which galaxies are currently "saying their last goodbyes?" That is, if we imagine that there are aliens living in these galaxies who hope to make contact with us, which galaxies are running up against their deadline right at this moment? A reasonable guess would be that the galaxies which are currently moving at the speed of light with respect to us (at a distance of 4,200 megaparsecs and redshift of 1.4, as discussed above) are at the "critical point" where any light they emit after now will never be able to reach us. Roughly speaking, this is correct, but a detailed calculation (such as the one contained in this paper) shows that for the simplest viable model of the universe's acceleration, it is actually galaxies at a distance of 4,740 megaparsecs and redshift of 1.69 that are just now reaching the critical point, while galaxies at a redshift of 1.4 are still emitting light that will eventually reach us.

The difference is due to a rather subtle fact: Even though the universe is "accelerating" in the sense that each galaxy moves faster as time goes on, the Hubble constant is actually decreasing with time -- in other words, the rate at which space is expanding, measured at a point which is at a fixed distance from us, gets smaller as time goes on. If we keep our eyes on an individual galaxy as it moves away from us, we will see it accelerate, but if we keep our eyes on a fixed point in space and watch many different galaxies go past that point, each galaxy's speed will be slower than the one before it. (As a very rough analogy, the universe behaves like a river with rapids. If you put a boat in the river and allow it to be carried by the flow, it will accelerate as it moves downstream and enters the rapids. But if you sit on the bank and measure the speed of the water at one location, it changes based on an entirely different set of factors -- for example, the rate at which the supply of water from upstream is changing. It is possible for the water speed at your location to decrease with time, even though each boat that you release accelerates as it heads into the rapids.) Because of this effect, if light is able to "swim against the tide" and remain at a roughly constant distance with respect to us (as would happen if it is emitted from a galaxy moving away from us at the speed of light), then as time goes on and the Hubble constant decreases, it will eventually be able to gain ground, "swim upstream" and traverse the necessary distance of space to reach us.>>

[alter-ego]

...In addition, light emitted by objects currently situated beyond a certain comoving distance (currently about 19 billion parsecs) will never reach Earth.

• This statement is not from the paper and looks plain wrong. What comoving distance (i.e. event horizon or particle horizon) is being referred to? Without more context, 19Gpc makes no sense - it is ~36% larger than the observable universe (aka particle horizon). Correctly stated in Wiki's first paragraph (quoted by Art), the particle horizon currently has a fundamental, limiting radius ~47Gly ( or ~14Gpc).
  → Per the current ΛCDM model, NOTHING beyond this radius has been or will ever be observable. It's a hard-wall boundary to anything observable beyond it. Of course, as time progresses, the particle horizon grows as space expands without new energy/objects added to the system.

• Regarding the Even[t] Horizon, for any object with z ≥ ~1.8, the light emitted today will never reach the Earth. Those higher-z objects are beyond our Event Horizon (comoving distance ≈ 16Gly) - Also not 19Gpc.

Granted...however, it is still not clear (to me at least) if the 2001 paper [cue the _Also sprach Zarathustra_ music] takes into account the fact that the Hubble [space] constant is decreasing in time such that ~14Gpc may not in the future represent the distance at which objects recede at the speed of light.

Well, if you're suggesting that the 19Gpc calculation did not include the slowing of the Hubble constant over time, that might be true. However, it isn't the paper, it's whoever wrote the Wiki paragraph. I've gone through the Cosmic expansion math in both the closed-form solution (Ω_VAC & Ω_M) in proper coordinates, and the more general comoving coordinate solution by integrating Friedmanns Eq. (includes radiation, Ω_R). I checked Loeb's results at Z=10, and agrees very well. His plots/calculations therefore must include the time dependence of H, and his approach is the closed-form one. In fact the Hubble constant is handled correctly in the paper. Unfortunately, the paper does not reveal many great analytical equations that very well describe cosmic expansion at times >10Myr after the Big Bang. As an example, the Hubble constant varies with time as:
 

Hubble Constand Time Dependence.jpg (4.37 KiB) Viewed 5401 times

 
 
 
 
 
Where Ho and Ωo are the Hubble constant and Vacuum energy fraction today. H_t and t are the Hubble constant at an arbitrary time, t, past or future). Of course, t=0 is the Big Bang.

[neufer]

• This statement is not from the paper and looks plain wrong. What comoving distance (i.e. event horizon or particle horizon) is being referred to? Without more context, 19Gpc makes no sense - it is ~36% larger than the observable universe (aka particle horizon). Correctly stated in Wiki's first paragraph (quoted by Art), the particle horizon currently has a fundamental, limiting radius ~47Gly ( or ~14Gpc).
  → Per the current ΛCDM model, NOTHING beyond this radius has been or will ever be observable. It's a hard-wall boundary to anything observable beyond it. Of course, as time progresses, the particle horizon grows as space expands without new energy/objects added to the system.

• Regarding the Even[t] Horizon, for any object with z ≥ ~1.8, the light emitted today will never reach the Earth. Those higher-z objects are beyond our Event Horizon (comoving distance ≈ 16Gly) - Also not 19Gpc.

Granted...however, it is still not clear (to me at least) if the 2001 paper [cue the _Also sprach Zarathustra_ music] takes into account the fact that the Hubble [space] constant is decreasing in time such that ~14Gpc may not in the future represent the distance at which objects recede at the speed of light.

Well, if you're suggesting that the 19Gpc calculation did not include the slowing of the Hubble constant over time, that might be true. However, it isn't the paper, it's whoever wrote the Wiki paragraph. I've gone through the Cosmic expansion math in both the closed-form solution (Ω_VAC & Ω_M) in proper coordinates, and the more general comoving coordinate solution by integrating Friedmanns Eq. (includes radiation, Ω_R). I checked Loeb's results at Z=10, and agrees very well. His plots/calculations therefore must include the time dependence of H, and his approach is the closed-form one. In fact the Hubble constant is handled correctly in the paper. Unfortunately, the paper does not reveal many great analytical equations that very well describe cosmic expansion at times >10Myr after the Big Bang. As an example, the Hubble constant varies with time as:

Where Ho and Ωo are the Hubble constant and Vacuum energy fraction today. H_t and t are the Hubble constant at an arbitrary time, t, past or future). Of course, t=0 is the Big Bang.

The Wiki paragraph was certainly misleading at best... but it referenced far more than a single article.

You seem to have far more familiarity with this subject than any of the rest of us here at the *Asterisk.

Would you mind telling us what your background on these matters is

[alter-ego]

…

Would you mind telling us what your background on these matters is

Long story short, my background in these matters is not formal. I have a degree in physics, an obsessive analytical tendency, a long career in science and industrial R&D, and a lifetime of interest in astronomy including GR and Cosmology portioned with opportunity, experience, hard work and some luck. Then I joined this forum where I often encounter challenging questions that I'm driven to answer, often turning into mini research projects.

Why did I pursue this project? I had a vision. A couple years ago, a redshift discussion prompted a desire to visualize of millions of objects expanding through space with their evolving redshifts over Gyrs of time. Objects with exaggerated brightness and color, changing in seconds that normally would take longer than the Universe is old. Realizing that vision led me to researching many papers, learning and practicing cosmoligical expansion and redshifts bit by bit. Two notable people, Prof. Ned Wright of UCLA, and astrophysicist, Dr. Tamara Davis, took time to look at, and comment on, my emailed questions. In my final summary package, Tamara said she had not seen a plot before with as much redshift verses time detail, and added she would not expect to see that kind of plot in many published papers. Cool!
So there's some background in a nutshell.

By the way, that plot provides me the visualization I wanted, but with a surprise. I expected to see all objects racing away from us with steadily increasing redshifts. That's not the case. Based on the present cosmological ΛCDM parameters for our accelerating universe,

• An observer today would see all objects with z < 2.3 speeding up (increasing redshifts) forever, while

• A few objects with z ≈ 2.3 have a redshift derivative, dz/dt, =0 showing a near-constant redshift for billions of years

• All objects with z >2.3 are decelerating relatively quickly (higher-z, faster deceleration) toward minima further into the future
  → Assuming a present redshift =50 ( emission time ≈ 50Myr), its minimum occurs about 11Gyrs from now at z = 23 (for the future observer)
  → For an past observer at the time acceleration begins to dominate (~7.3Gyr after the Big Bang), all objects would be slowing down

Mission accomplished.

[neufer]

Mission accomplished.

I, personally, prefer placing that particular banner at the top of my posts.

Do you have an opinion about the current discrepancies in the determination of the Hubble Constant?

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Estimated values of the Hubble constant, 2001–2019. Estimates in black represent calibrated distance ladder measurements which tend to cluster around 73 km/s/Mpc, red represents early universe CMB/BAO measurements with ΛCDM parameters which show good agreement on a figure near 67 km/s/Mpc, while blue are other techniques, whose uncertainties are not yet small enough to decide between the two.

[alter-ego]

>>>

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Estimated values of the Hubble constant, 2001–2019. Estimates in black represent calibrated distance ladder measurements which tend to cluster around 73 km/s/Mpc, red represents early universe CMB/BAO measurements with ΛCDM parameters which show good agreement on a figure near 67 km/s/Mpc, while blue are other techniques, whose uncertainties are not yet small enough to decide between the two.

Yeah, I do.
Within the present redshift range (z < ~7) of measurements, I don't believe astronomers will rectify the Hubble tension. I think they'll only continue to increase the presently almost-5σ error between CMB-based measurements and measurements using the "local" galaxies and quasars visible to date. I'd like to see redshift measurements out to z = 20, but that will take the JSWT. Either way, if the higher-z redshift measurements tend to roll over, tending to approach the Planck results, or remain in line with the present greater Hubble constant, then there is likely some missing physics .

Three recent survey results are increasing this tension and are notable in my opinion:

• Results from the new SDSS EBOSS 20-year survey sampling 2 million local objects (galaxies and quasars filling 6Gyr to 11Gyr look-back time), mapped more precisely the growth of BAOs (characteristic separation) from the CMB origin through large scale structure of the local universe. Claiming to have the "most accurate expansion history measurements over the widest-ever range of cosmic time", the Hubble constant is closer the Planck result (between 67.4 and 69.0 km/s/Mpc)
  → It's particularly interesting that expansion of BAOs from CMB to local objects appears consistent with Planck, but the Hubble constant determined from local redshifts is not.

  

• The Kilo-Degree Survey (KIDS) mapped 31 million galaxies in 1000 square degrees to evaluate the clumping of gravitating matter on a finer scale.
  → Analysis yields the distribution is about 8.3% smoother than theory predicts

• The Dark Energy Survey has just submitted their results.
  → Analysis yields the clumping is 5.5% less

The tension is certainly building, and is looking more and more like something's got to give, more data is needed.

Starting three or so years from now, the European Space Agency’s Euclid mission and the Vera C. Rubin Observatory will greatly improve on current surveys, both in sensitivity and in sky coverage. Eventually, it will become clear whether or not we’ll have to discard our cherished theoretical model of the universe.

I must admit, these are exciting times.