Ask Me Anything Ask Dr. Joe - Now WEEKLY!

[DrJoePesce]

Dear Dr Joe Pesce,

Appreciate your commitment to answer questions at this forum and your willingness to put your expertise at our service! That being said, I find some aspects of your answer to me (#120 in this thread) are confusing. Your explanation that light from the most distant galaxy was originally ultraviolet but that by the time it reached us, because of the expansion of the Universe, it had red-shifted into the microwave and infra-red range, while it might hold good for that particular Galaxy does not seem to hold good for others. For instance, the Galex observatory that was in operation from 2003 to 2010 operated solely in the ultra-violet range, yet was able to map stars and galaxies that were 10 billion light years distant. (Hopefully, the James Webb Telescope will be able to do even better.) The significant point is that these observations were made in the ultraviolet range, not ultraviolet that had morphed into the infrared or microwave frequencies. Surely, if the Universe were expanding, it would do so at a uniform rate? Not haphazardly. If the Universe were indeed expanding at the rates being widely proclaimed then it should be impossible to see in the ultraviolet to distances of billions of light years.
Would it be possible to ascertain how these two conflicting theories are being so widely published and what is the reasoning behind such divergent facts?

Jzz, thanks again for allowing me to clarify this important aspect of astrophysics.

The expansion isn’t happening for just some objects, nor is the shifted wavelengths only happening for a subset of objects: the expansion is universal (everywhere) and every object emitting electromagnetic radiation is affected in the same way (different amounts for different distances traveled, of course).

For all objects emitting ultraviolet radiation (say) 13 billion years ago, ALL of those ultraviolet photons have lost energy and are now emitting in the millimeter part of the electromagnetic spectrum, after traveling to us (13 billion light years from the source), or any other location 13 billion light years from the source.

You mention Galex observations of distant objects in the ultraviolet. Galex is observing those ultraviolet photos today. At the source (say at 10-12 billion light years away) those photons were emitted at hard x-rays or gamma-rays. In traveling through the expanding universe (see my response to your previous question), their wavelengths have been stretched (because space is being stretched from under them), lowering their energy (longer wavelengths means lower energy) and are now in the ultraviolet.

So whenever we observe an object today, in whatever region of the electromagnetic spectrum, if we want to know what those photons were at the source, we need to “correct” for the distance traveled.

I’ll make up an example, as a thought experiment: Our Sun emits electromagnetic radiation from the radio to the gamma-rays. If I magically place the Sun at 10 billion light years away, and then step back and observe the Sun, I am seeing those photos as they have been redshifted by their long journey (through an expanding universe). The radio and infrared photons are redshift almost to zero energy (at least to very long wavelength radio). The higher energy photons, say from visible to gamma-ray at the point they left the Sun, would be observed here on Earth from radio (for the source visible-light photons) to visible (for the source gamma-ray photons).

In my example to your previous question in this thread, for the most distant galaxy. ALMA is observing, specifically, a spectral line created by oxygen in the ultraviolet portion of the spectrum (at the source). That spectral line has been redshifted into ALMA’s viewing window in the millimeter part of the spectrum. It’s that spectral line that allows us to measure the distance. There are photons throughout the electromagnetic spectrum emitted by the source that are all redshifted. Some out of ALMA’s range, and some in it – depending on their initial wavelengths.

I hope this makes sense.

Dr. Joe

 

[Catastrophe]

Dear Dr Joe,

We have a lot of discussion around here from time to time. Much is involved with division by zero. What would you consider to be a good general scientific working definition?

Mine is
"∞ is just a useful mathematical fiction, and one which does not have any corresponding substance in reality.

Beyond that one can write books about speculation, assumption, and imagination."

Nothing metaphysical

Cat

 

[Jzz]

For all objects emitting ultraviolet radiation (say) 13 billion years ago, ALL of those ultraviolet photons have lost energy and are now emitting in the millimeter part of the electromagnetic spectrum, after traveling to us (13 billion light years from the source), or any other location 13 billion light years from the source.

Dear Dr Joe,
My sincere and heart-felt thanks for the time and trouble taken in answering my question. My research on the net into the subject of redshift reveals the following. The vast majority of comments on the subject state that it is possible to take pictures of Galaxies that are billions of light years distant (10 billion light years distant for example) by lengthening the time of exposure. For instance, to image the Hubble Deep field took 100 hours of exposure time. That having been said, the article also states that for really distant objects the expansion of the Universe redshifts the frequencies into the infrared region, so that imaging has to be done at the infrared range and not in the optical range and then extrapolated to give the correct colours. https://esahubble.org/science/deep_fields/

Interesting but equally interesting is the following:

Edwin Hubble: In order to make this concept clearer, it is necessary to go back to the founding of the Big Bang theory. Every theory of the Universe, whether static or dynamic, would have remained in the realms of fantasy if it had not been for one man. That man was Edwin Hubble, the man who first discovered that the Universe was expanding. Edwin Hubble ties in closely with the Big Bang Theory. Therefore, his discovery needs to be got into, in a little more detail. Hubble found that the Universe was expanding at a steady rate and that the further away we looked the faster the rate of expansion. How did Hubble measure the rate of expansion of the Universe? He used the Doppler redshift. Relativistic Doppler was not in common use at the time Hubble made his discoveries. How could have Hubble use the Doppler redshift, it works only for sound? In fact, sound and light are very similar both travel at a constant speed. Sound travels at a constant speed as long as the medium it is travelling through does not change. The speed of light is also constant. Therefore, the equation Hubble used to determine the expansion of the Universe was not the “relativistic” Doppler redshift. It was just the plain Doppler redshift according to which:

Observed frequency = ((speed of light / (speed of light + speed of light source)) x original frequency.

As with special relativity the Doppler redshift works flawlessly at low speeds but relativistic Doppler redshift is needed when the sources are moving at near light speeds. Take for instance a Galaxy that is emitting blue light with a frequency of 6.66 x 10^14 Hz and a wave length of 450 nm. Suppose that it is two megaparsecs away from earth what would be its redshift according to Hubble’s law?

F_o = (c/(c + v) f which gives ((2.997 x 10^8/ (2.997 x 10^8 + 146)x (6.66. x 10^14) = 6.659 Hz

(Nothing much at this distance but noticeable further out, I suppose. )

Age of the Universe: The result that Hubble derived from his observations of the red shift of stars and Galaxies, was that the Universe was expanding at the rate of approx. 70 km/s per megaparsec. A megaparsec is a distance of 3.6 million light years. This is the data that was used to calculate the age of the Universe. Since the Universe expands at a steady rate it is possible to determine the age of the Universe by simply extrapolating an arbitrary distance to calculate when the speed of expansion equalled the speed of light. It was assumed that this would be the point at which the Universe formed, or the Big Bang event took place. The time thus determined was approx. 13.8 billion years ago.

Given that Hubble’s law stating that the velocity of recession of distant galaxies from our own is proportional to their distance from us, seems to hold good. Why is it necessary to introduce concepts such as expansion of the Universe and space time needed and all the different types of redshifts.

The present theory seems to be of no practical use. Only mathematicians, and very esoteric mathematicians at that, gain any benefit from this theory. Apart from controlling peoples thoughts, ideas and imagination and enjoying the power that this bestows, there is absolutely no credible basis for any of these theories. It should be open for debate. What is the point of talking about a Universe that was born 13.8 billion years ago but has since expanded to about 94.5 billion light years across? Of what possible interest can result from such facts? Are these facts even true?

 

[billslugg]

Jzzz said:
"F_o = (c/(c + v) f which gives ((2.997 x 10^8/ (2.997 x 10^8 + 146)x (6.66. x 10^14) = 6.659 Hz "

I assume by "146" you meant 146 kilometers per second (which is 1.46x10^5 meters per second.
In that case your result should be .9995 times the original frequency or 6.657x10^14 Hz, not 6.659 Hz.
Still in the blue region, as you mentioned.

Jzzz said:
"... there is absolutely no credible basis for any of these theories."

There is a thread in the Cosmology forum that lists 26 lines of evidence supporting the Big Bang theory. You have not commented there so why don't you go over now and dispute them?

Big Bang Bullets II

Here is a tweaked and more colorful version of the prior arguments favoring Big Bang Theory. [Comments welcome, of course.] [/url ] [url=https://postimg.cc/1fnG4G7L]

forums.space.com

 

[Jzz]

There is a thread in the Cosmology forum that lists 26 lines of evidence supporting the Big Bang theory. You have not commented there so why don't you go over now and dispute them?

Thanks for the correction to my maths. You obviously have the wrong take on what I am saying. I believe in the Big Bang theory , just not in the relativistic approach to it. Hubble's findings were purely empirical and cannot be disputed.
In any case with the James Webb Telescope coming online and with its almost exponential increase in performance over the Hubble space telescope in the infrared range, all of these disputes should soon be resolved.

 

[Catastrophe]

Re: the Hubble Inconstant:

https://imgur.com/a/zdbFJY6

View: https://imgur.com/a/zdbFJY6



This assumes the retro extension must pass through the origin, which forces the conclusion that there is a zero velocity (single point), which begs the question. IMHO, it is not the data at fault (though far from accurate) but the forced extrapolation.

To me, the line of best fit is a fiction, or, at best, a vague suggestion.


Cat

 

[Catastrophe]

Here is another. To me, it is a vague suggestion of a correlation.

https://imgur.com/a/VD72OMp

View: https://imgur.com/a/VD72OMp#MMfBdOg


Cat

 

[Helio]

Here is another. To me, it is a vague suggestion of a correlation.

https://imgur.com/a/VD72OMp

View: https://imgur.com/a/VD72OMp#MMfBdOg


Cat

Nice plot!

When reading about the SN studies by the two teams in the late 1990’s to nail the Ho constant — they discovered DE from this effort— it was enjoyable that the team with astronomers (Harvard) beat the team of physicists (Berkeley) who were given more telescope time. But the astronomers, with fewer data points, were able to be more accurate since they used multiple filters to better refine their observations.

 

[Jzz]

Here is another. To me, it is a vague suggestion of a correlation.

This is a very interesting observation that Catastrophe has made, and one which I would like to request Dr. Joe Pesce to help resolve. Considering the sheer magnitude of the Universe and the amazing correlation of the data, the Hubble - Lemaitre Law and the conclusions that have been drawn from that data are perfectly valid. However, there is one aspect of all this that puzzles me greatly. Granted, that the data clearly demonstrates a relation between the distance of a star or galaxy from our position and the speed at which it is moving. What puzzles me is that the further away a star is, the further away in time it is from us and the closer to the Big Bang event the star is. Therefore, it should be natural that those stars that are further away and closer to the Big Bang expansion event, should move faster and this is exactly what the data demonstrates. In fact, stars and Galaxies that are extremely distant, in the 13 billion light year range for instance, should be moving at speeds that are close to the speed of light and again this is just what the data shows to be the case.

My question is how can this predictable course of events lead to conclusions that the Universe is still expanding and that this expansion is taking place at close to or in most cases exceeding light speeds? Surely, given the huge distances involved and the inherent uncertainties in such a situation; to make such bombastic claims is premature?

Infact, if the Big Bang theory is correct, there should be a point where what is detected is enormous amounts of energy with anomalous frequencies and where redshifts are inapplicable.

 

[DrJoePesce]

Greatly appreciate the clarification. Although I have to state that using the relativistic red-shift to explain the expansion of the Universe still does not sit to well with me.

Hi Jzz - the relativisitic redshift is an outcome of the expansion. We can measure velocity and direction independently. Then, once we have ascertained that the universe is expanding, the redshift, z, can be defined and determined.

Dr. Joe

 

[billslugg]

Redshift is not only observed at the edge of the universe it is present at all distances. This is why we can extrapolate up to today and make the well founded assumption that expansion is still going on.

 

[Helio]

I will add that redshift is, somehow, the result of travel through expanding space time, so not Doppler. We are observing light that originated in regions traveling much faster than c, something Doppler can’t address.

 

[DrJoePesce]

Dear Dr Joe,

We have a lot of discussion around here from time to time. Much is involved with division by zero. What would you consider to be a good general scientific working definition?

Mine is
"∞ is just a useful mathematical fiction, and one which does not have any corresponding substance in reality.

Beyond that one can write books about speculation, assumption, and imagination."

Nothing metaphysical

Cat

There are mathematical concepts that may or may not have an existence in reality, but are necessary none-the-less.

True, there is probably nothing in the universe that physically equates to infinity. And even if expansion continues for ever, we will be approaching it.

 

[Helio]

I wish asymptotes were easier to spell.

 

[DrJoePesce]

I will add that redshift is, somehow, the result of travel through expanding space time, so not Doppler. We are observing light that originated in regions traveling much faster than c, something Doppler can’t address.

Yes, cosmological redshift is caused by the fabric of space stretching as the photon flies through it - stretching the photon's wavelength. Stretched wavelength equals lower energy photon (and a "redder" , in this case "redder" implying lower energy).

 

[Helio]

Yes, cosmological redshift is caused by the fabric of space stretching as the photon flies through it - stretching the photon's wavelength. Stretched wavelength equals lower energy photon (and a "redder" , in this case "redder" implying lower energy).

Yes. I think Slipher, Lemaitre and maybe everyone else in the 1930’s favored Doppler, but maybe not once regions were found faster than c.

 

[DrJoePesce]

Dear Dr Joe,
My sincere and heart-felt thanks for the time and trouble taken in answering my question. My research on the net into the subject of redshift reveals the following. The vast majority of comments on the subject state that it is possible to take pictures of Galaxies that are billions of light years distant (10 billion light years distant for example) by lengthening the time of exposure. For instance, to image the Hubble Deep field took 100 hours of exposure time. That having been said, the article also states that for really distant objects the expansion of the Universe redshifts the frequencies into the infrared region, so that imaging has to be done at the infrared range and not in the optical range and then extrapolated to give the correct colours. https://esahubble.org/science/deep_fields/

Interesting but equally interesting is the following:

Edwin Hubble: In order to make this concept clearer, it is necessary to go back to the founding of the Big Bang theory. Every theory of the Universe, whether static or dynamic, would have remained in the realms of fantasy if it had not been for one man. That man was Edwin Hubble, the man who first discovered that the Universe was expanding. Edwin Hubble ties in closely with the Big Bang Theory. Therefore, his discovery needs to be got into, in a little more detail. Hubble found that the Universe was expanding at a steady rate and that the further away we looked the faster the rate of expansion. How did Hubble measure the rate of expansion of the Universe? He used the Doppler redshift. Relativistic Doppler was not in common use at the time Hubble made his discoveries. How could have Hubble use the Doppler redshift, it works only for sound? In fact, sound and light are very similar both travel at a constant speed. Sound travels at a constant speed as long as the medium it is travelling through does not change. The speed of light is also constant. Therefore, the equation Hubble used to determine the expansion of the Universe was not the “relativistic” Doppler redshift. It was just the plain Doppler redshift according to which:

Observed frequency = ((speed of light / (speed of light + speed of light source)) x original frequency.

As with special relativity the Doppler redshift works flawlessly at low speeds but relativistic Doppler redshift is needed when the sources are moving at near light speeds. Take for instance a Galaxy that is emitting blue light with a frequency of 6.66 x 10^14 Hz and a wave length of 450 nm. Suppose that it is two megaparsecs away from earth what would be its redshift according to Hubble’s law?

F_o = (c/(c + v) f which gives ((2.997 x 10^8/ (2.997 x 10^8 + 146)x (6.66. x 10^14) = 6.659 Hz

(Nothing much at this distance but noticeable further out, I suppose. )

Age of the Universe: The result that Hubble derived from his observations of the red shift of stars and Galaxies, was that the Universe was expanding at the rate of approx. 70 km/s per megaparsec. A megaparsec is a distance of 3.6 million light years. This is the data that was used to calculate the age of the Universe. Since the Universe expands at a steady rate it is possible to determine the age of the Universe by simply extrapolating an arbitrary distance to calculate when the speed of expansion equalled the speed of light. It was assumed that this would be the point at which the Universe formed, or the Big Bang event took place. The time thus determined was approx. 13.8 billion years ago.

Given that Hubble’s law stating that the velocity of recession of distant galaxies from our own is proportional to their distance from us, seems to hold good. Why is it necessary to introduce concepts such as expansion of the Universe and space time needed and all the different types of redshifts.

The present theory seems to be of no practical use. Only mathematicians, and very esoteric mathematicians at that, gain any benefit from this theory. Apart from controlling peoples thoughts, ideas and imagination and enjoying the power that this bestows, there is absolutely no credible basis for any of these theories. It should be open for debate. What is the point of talking about a Universe that was born 13.8 billion years ago but has since expanded to about 94.5 billion light years across? Of what possible interest can result from such facts? Are these facts even true?

Dear Jzz,

Yes, the most distant objects have light shifted to the IR and longer wavelengths. This is one reason the James Webb Space Telescope will be useful and important. Also note the most distant galaxy we talked about in previous posts was observed in the radio (millimeter) portion of the spectrum by ALMA.

Hubble’s observation that galaxies are all moving away from us, and at higher velocities the more distant they are implies an expanding universe. That is one of the key observables pointing us to universal expansion.

Note also that the more distant the object from us the speeds are, as you say, approaching relativistic velocities. This is why relativity is necessary.

Incidentally, why are the objects moving faster the more distant they are from us? In the classroom, I have a transparency on which I made dots representing galaxies. I then “stretch” the transparency sheet (with technology) in 2-D (it’s a sheet of transparency, after all). All the dots move away from each other. Those more distant move a larger distance. Hence, in a given time, they have to be moving faster. And this is what’s happening with the universe – it’s as simple as that. (If you prefer the analogy of a raisin cake, baking in the oven, think of the raisins spread throughout the cake matrix. Before we put it in the oven, there are two rainsins close to the center of the cake, which is 1cm thick. After we back the cake for 10 mins, the two raisins are still in the center, slightly further apart, but the raisin at the edge – which was 1 cm away from the center before backing – is now 6cm away. It’s moved much faster than the raisins in the center….

Dr. Joe

 

[DrJoePesce]

Re: the Hubble Inconstant:

https://imgur.com/a/zdbFJY6

View: https://imgur.com/a/zdbFJY6



This assumes the retro extension must pass through the origin, which forces the conclusion that there is a zero velocity (single point), which begs the question. IMHO, it is not the data at fault (though far from accurate) but the forced extrapolation.

To me, the line of best fit is a fiction, or, at best, a vague suggestion.


Cat

Thanks for the graph, Cat. That is one made for the Earth, of course, which is at zero. We could make one of these from the perspective of M87 or the Andromeda galaxy and should get the same result: v = H * D, with H giving the slope, of course.

Dr. Joe

 

[DrJoePesce]

This is a very interesting observation that Catastrophe has made, and one which I would like to request Dr. Joe Pesce to help resolve. Considering the sheer magnitude of the Universe and the amazing correlation of the data, the Hubble - Lemaitre Law and the conclusions that have been drawn from that data are perfectly valid. However, there is one aspect of all this that puzzles me greatly. Granted, that the data clearly demonstrates a relation between the distance of a star or galaxy from our position and the speed at which it is moving. What puzzles me is that the further away a star is, the further away in time it is from us and the closer to the Big Bang event the star is. Therefore, it should be natural that those stars that are further away and closer to the Big Bang expansion event, should move faster and this is exactly what the data demonstrates. In fact, stars and Galaxies that are extremely distant, in the 13 billion light year range for instance, should be moving at speeds that are close to the speed of light and again this is just what the data shows to be the case.

My question is how can this predictable course of events lead to conclusions that the Universe is still expanding and that this expansion is taking place at close to or in most cases exceeding light speeds? Surely, given the huge distances involved and the inherent uncertainties in such a situation; to make such bombastic claims is premature?

Infact, if the Big Bang theory is correct, there should be a point where what is detected is enormous amounts of energy with anomalous frequencies and where redshifts are inapplicable.

Hi Jzz, I’m not sure what the “bombastic claim” is. That the universe is still expanding?

First of all, every galaxy we observe (apart from the most local ones affected by the gravity of our Milky Way) is moving away from us with a velocity that increases with increasing distance (and note we can determine distances independent of redshift – at least for some objects). This is an observational fact. So we explicitly see expansion everywhere (space and time).

What could affect expansion? Mass (or, better, density). Indeed, up until 15 years ago or so, this was one of the most important questions in cosmology: What is the mass density of the universe and is it sufficient to halt expansion. This is one reason dark matter is so important – if it exists and we haven’t taken it into account, then we are tallying the mass in the universe incorrectly. From the mass-density perspective, there are three possible outcomes for universal expansion: 1) expanding forever (not sufficient mass to stop expansion), 2) expanding forever but at an ever-decreasing rate, tending toward zero but never getting there (mass at a critical value, but still not enough to stop expansion), or 3) expansion stopping and then the universe collapses (mass sufficient to halt expansion and reverse it). All observations were indicating we were in regime #2: expanding forever but at an ever-decreasing rate.

With the advent of the dark energy discovery, that is now almost a moot point: Expansion has accelerated, and we will continue to expand forever. There is just not enough mass in the universe to stop this (and matter can’t be created so no prospects of more mass in the future).

Does this help?

Dr. Joe

 

[Catastrophe]

Thanks for the graph, Cat. That is one made for the Earth, of course, which is at zero. We could make one of these from the perspective of M87 or the Andromeda galaxy and should get the same result: v = H * D, with H giving the slope, of course.

Dr. Joe

Thank you, Dr Joe, for your further comment. Would it be fair to say that there is a discontinuity in the V = H * D equation close to the origin (wherever this is selected) because at low values, close to the origin, there will be attraction between objects, due to gravitation, as we know does occur in the Local Group, including our merger with the Andromeda Galaxy in about 4 billion years. Thus V becomes negative (approach v expansion).

This being the case, and granted that the 'singularity' concept seems slightly muddied, does this not raise the nexus idea (without any elaboration whatsoever) to the same level of doubt as the 'singularity'.

In short, does not the break down of the Hubble Equation close to the origin (and t = 0) due to gravitational attraction, and the breakdown of science to operate under conditions of division by zero, make it inadmissible to extrapolate the V = H * D graph all the way back to zero? In fact, the graph shows some points to the right of the origin ringed, and noted that "some galaxies may be moving towards us".

https://imgur.com/a/gAgswPs

View: https://imgur.com/a/gAgswPs

I have just noticed that the graph does show these points as having negative velocities.

Please may I remind you that I am totally in agreement with BBT, as starting slightly above the origin. I just do not believe that it is admissible to make the line on the graph hit the origin. I am purely imagining, without any experimental evidence, that, if the 'Hubble line' does not reach the origin, then it is just as admissible to suggest a 'nexus' as to suggest a 'singularity' at t = 0. I am not even thinking of anything beyond the nexus, anymore than science can think of anything 'real' about a singularity.

Thanks again, very much indeed, for spending some time with us and answering our questions.

Cat

 

[DrJoePesce]

Thank you, Dr Joe, for your further comment. Would it be fair to say that there is a discontinuity in the V = H * D equation close to the origin (wherever this is selected) because at low values, close to the origin, there will be attraction between objects, due to gravitation, as we know does occur in the Local Group, including our merger with the Andromeda Galaxy in about 4 billion years. Thus V becomes negative (approach v expansion).

This being the case, and granted that the 'singularity' concept seems slightly muddied, does this not raise the nexus idea (without any elaboration whatsoever) to the same level of doubt as the 'singularity'.

In short, does not the break down of the Hubble Equation close to the origin (and t = 0) due to gravitational attraction, and the breakdown of science to operate under conditions of division by zero, make it inadmissible to extrapolate the V = H * D graph all the way back to zero? In fact, the graph shows some points to the right of the origin ringed, and noted that "some galaxies may be moving towards us".

https://imgur.com/a/gAgswPs

View: https://imgur.com/a/gAgswPs

I have just noticed that the graph does show these points as having negative velocities.

Please may I remind you that I am totally in agreement with BBT, as starting slightly above the origin. I just do not believe that it is admissible to make the line on the graph hit the origin. I am purely imagining, without any experimental evidence, that, if the 'Hubble line' does not reach the origin, then it is just as admissible to suggest a 'nexus' as to suggest a 'singularity' at t = 0. I am not even thinking of anything beyond the nexus, anymore than science can think of anything 'real' about a singularity.

Thanks again, very much indeed, for spending some time with us and answering our questions.

Cat

Hi Cat - one quick thing: The origin is NOT the big bang event. It's the location of Earth (as the observer). You are correct, the universal expansion is happening everywhere, but it is currently most noticeable on the largest scales (the expansion rate is kilometers per second per megaparsec (1 million parsecs, or about 3.3 million light years). So for the most nearby galaxies, influenced by the gravity of the Milky Way (and vice versa), the expansion is hidden by much larger local gravitational effects. That's why those close-by objects are circled in the graph.

Dr. Joe

 

[DrJoePesce]

Hey Astronomy Enthusiasts!

It's Black Hole week - one of my favorite topics!! These enigmatic objects, first predicted over a century ago, have captivated humans ever since.

Now we are in a golden age of astrophysics, providing us with insights into these fascinating phenomena in a way never before possible.

From colliding black holes producing prodigious gravitational waves, to monsters in the centers of galaxies (one of which, M87, we saw for the first time in 2019), to big black holes in the early universe, we are learning more and more about them every day.

You can explore these most extreme of astrophysical objects with me here on the forum this week—send your questions!—and by visiting https://beta.nsf.gov/blackholes for images, video, virtual backgrounds, and more.

Follow us on the forum, and watch NSF's social media channels throughout the week for more conversations:

https://www.facebook.com/US.NSF/

https://twitter.com/NSF

https://www.instagram.com/nsfgov/

https://www.youtube.com/user/VideosatNSF

Dr. Joe

 

[Orion]

Dr. Joe:

What is the minimum amount of humans be for a future colony mission?

 

[Jzz]

Hi Jzz, I’m not sure what the “bombastic claim” is. That the universe is still expanding?

First of all, every galaxy we observe (apart from the most local ones affected by the gravity of our Milky Way) is moving away from us with a velocity that increases with increasing distance (and note we can determine distances independent of redshift – at least for some objects). This is an observational fact. So we explicitly see expansion everywhere (space and time).

Does this help?

Dr. Joe

Thank you Dr Joe! Of course, it always helps having an informed source, apart from that it is a pleasure to discuss these questions without aggressive or intimidating influences. I hope you will admit that it is peculiar, especially if we take the Socratic route:
Q) Are stars different from Galaxies?
Ans) Yes of course they are, a star is an individual entity while a galaxy is made up of billions and billions of stars.
Q) That being so and taking our own Galaxy as an example we find that the stars are separated by distances of light years (1 light year is 10,000,000,000,000 km approx.) This being the case isn't it strange that the stars within these gigantic galaxies are not expanding away from each other? ?? The present wisdom states that :

"Space is expanding from the Big Bang and the acceleration of dark energy. But the objects embedded in space, like planets, stars, and galaxies stay exactly the same size. As space expands, it carries galaxies away from each other. From our perspective, we see galaxies moving away in every direction. "

Quote from Universe today.

It must indeed be a peculiar form of expansion especially since it is based on these assumptions that the theory that the Universe is expanding away at several times the speed of light is based.

 

[DrJoePesce]

Dr. Joe:

What is the minimum amount of humans be for a future colony mission?

Hi Orion - I'm afraid I don't know the answer to that off the top of my head. Let me do some research. I believe there has been quite a bit of writing on this....

Dr. Joe