Evidence that the key assumption made in discovery of dark energy is in error • Hacker News Board

Re: Evidence that the key assumption made in discovery of dark energy is in error

#231

Post by bananabreakfast » Tue, Jan 07, 2020, 7:04 PM UTC

bentona wrote: post #91

bananabreakfast wrote: post #77

Earlier quoted context omitted.

Speaking from the pov as a former astrophysicist, that definition of redshift is not quite accurate. While we have observed the universe expanding since Edwin Hubble published his findings 90 years ago, it was in fact the Friedmann equations produced 10 years earlier that established how it would work within the context of general relativity. The Universe is expanding at every known point at the same time at the same…

Seems like a very detailed yet approachable explanation, wondering about your thoughts? https://www.youtube.com/watch?v=7UNLgPIiWAg

Pretty interesting video. The big takeaway is definitely the end where he talks about the reality of the scientific method.

Unlike math, you can never prove that something is correct and true. You can only prove that reality likely does not fit a given theoretical model to a vanishing probability. The models we have, we only have because they have not been proven to have a vanishingly small likelihood of explaining physical phenomena.

Btw, this is what frustrates everyone about string theory. It's mathematically correct and quite elegant, but extremely difficult to gather any observational data that could prove it unlikely or another theory unlikely compared to it, leaving it in a sort of scientific limbo.

Astrophysics is hard because it is an observational science only. You cannot run your own experiments so you must simply gather as much observational data as possible. It is really, really hard to gather significant data about the universe at the largest scales. The video talked about hundreds of observations, but in a universe with more whole galaxies than grains of sand on earth there are implicitly a huge number of assumptions about how representative any data set is.

The numbers in astrophysics are so mind-meltingly enormous that often unimaginable and unintuitive consequences become regular occurrences.

This new data should obviously not be ignored, but it also shouldn't be interpreted as "disproving" anything. That is journalistic attention grabbing. Rather the results are simply some experimental physicists saying to theoretical ones: "here, look at these interesting new data about important data sources. It reduces our uncertainty about one specific thing. Please incorporate this into your cosmology theories"

Re: Evidence that the key assumption made in discovery of dark energy is in error

#232

Post by Certhas » Tue, Jan 07, 2020, 7:11 PM UTC

SAI_Peregrinus wrote: post #221

Koshkin wrote: post #186

Earlier quoted context omitted.

> relative to the rubber This makes me uneasy about your analogy. It’s like saying “relative to space” which is nonsense. Also, why would space limit itself to only expanding outside a physical body?

> Also, why would space limit itself to only expanding outside a physical body? It doesn't, it's just that the forces within the body (the spring in the analogy) are stronger than the expansion. The "relative to space" bit is the nonsense part of their analogy, but I'm not sure of a clearer way to describe things than what they did. It's not my field.

Maybe a better way to phrase it would have been moving over the rubber/space? Or through the rubber/space? It really is motion relative to the underlying manifold though. Let's say there was such a thing as a gravitational soliton, that is, a stable ripple in spacetime, then if you are on the same spot as this wrinkle as expansion starts you will move away from it. You move relative to a structure made from spacetime. This was my field for many years. :-)

The mathematically precise statement is that you are not moving on a geodesic. You are therefore experiencing acceleration (the objects that are not bound and have increasing distance are on geodesics and do not experience acceleration).

Re: Evidence that the key assumption made in discovery of dark energy is in error

#233

Post by bananabreakfast » Tue, Jan 07, 2020, 7:23 PM UTC

mannykannot wrote: post #200

bananabreakfast wrote: post #77

Earlier quoted context omitted.

Speaking from the pov as a former astrophysicist, that definition of redshift is not quite accurate. While we have observed the universe expanding since Edwin Hubble published his findings 90 years ago, it was in fact the Friedmann equations produced 10 years earlier that established how it would work within the context of general relativity. The Universe is expanding at every known point at the same time at the same…

I am confused, as a lot of sources (e.g. [1]) make a distinction between doppler and cosmological redshift. When you say "change our relativistic reference frame to match a distant galaxy's", do you mean as that frame was when the photon was emitted, or as it is when it is detected here? (if that is a well-defined distinction under relativity?) [1] https://calgary.rasc.ca/redshift.htm

So that source draws a line between cosmological redshift and doppler redshift that is so misleading I'd say it's wrong. They are in fact equivalent [1] and basically just depend on your reference frame.

Check out the wikipedia page on Hubble's Law [2] for more.

[1] https://arxiv.org/abs/0808.1081 [2] https://en.wikipedia.org/wiki/Hubble%27s_law

Re: Evidence that the key assumption made in discovery of dark energy is in error

#234

Post by bananabreakfast » Tue, Jan 07, 2020, 7:24 PM UTC

cygx wrote: post #149

bananabreakfast wrote: post #77

Earlier quoted context omitted.

Speaking from the pov as a former astrophysicist, that definition of redshift is not quite accurate. While we have observed the universe expanding since Edwin Hubble published his findings 90 years ago, it was in fact the Friedmann equations produced 10 years earlier that established how it would work within the context of general relativity. The Universe is expanding at every known point at the same time at the same…

However, if you were to change our relativistic reference frame to match a distance galaxy's [...] In general, you can't do that: There's no distance parallelism in curved spacetimes...

Well, curved spacetimes only matter when you're dealing with gravitational redshift, which we aren't in this situation.

Re: Evidence that the key assumption made in discovery of dark energy is in error

#235

Post by cygx » Tue, Jan 07, 2020, 7:38 PM UTC

bananabreakfast wrote: post #234

cygx wrote: post #149

Earlier quoted context omitted.

However, if you were to change our relativistic reference frame to match a distance galaxy's [...] In general, you can't do that: There's no distance parallelism in curved spacetimes...

Well, curved spacetimes only matter when you're dealing with gravitational redshift, which we aren't in this situation.

Why would you think that? FLRW spacetime isn't Minkowski space, thus has nonzero curvature.

Re: Evidence that the key assumption made in discovery of dark energy is in error

#236

Post by shpeak » Tue, Jan 07, 2020, 7:41 PM UTC

Certhas wrote: post #230

shpeak wrote: post #222

Earlier quoted context omitted.

We can directly measure distance to stars which are near us, using trigonometry and waiting half of year for Earth to make half of distance around Sun. Distance to thousands of stars is already measured with ±5% precision.[0] Yes, it's true that at least some stars, which are near to us, are moving away from us. However, it can be explained in number of ways, without inventing of Bing Bang or other epic events. We ca…

What's your point? Do you want me to explain the basics of GR and Cosmology, including the vast amounts of evidence we have for everything in HN comments? Do you think you are pointing out subtle errors in reasoning that none of the tens of thousands of physics students since the early twentieth century spotted? You don't have the decency to try to learn the basics but presume to lecture me/expect explanations? Take…

I'm trying to point out, that current evidence can be explained in different way: by kind of Tired Light Theory. TLT plays well with Pilot Wave Theory and it doesn't need epic events in the past just few galactic hours ago. I'm familiar with evidence used by expanding Universe theories and with problems in them.

TLT predicts that value of "Speed of expanse" will be different when measured using different methods or frequencies, and it is: https://en.wikipedia.org/wiki/Hubble%27s_law#Measured_values... .

TLT predicts that there is much more stars, but we cannot see them yet because they are too dim, but more powerful telescopes will be able to pickup them. https://en.wikipedia.org/wiki/Tolman_surface_brightness_test https://www.nasa.gov/feature/goddard/2016/hubble-reveals-obs...

TTL predicts stars and galaxies older than BB (because of no BB), and they are found: https://en.wikipedia.org/wiki/List_of_the_most_distant_astro...

TTL predicts that Cosmic Background Radiation is just radiation of distant objects with Z=1000 .

And so on.

In short, mainstream theories doesn't hold against new data.

Re: Evidence that the key assumption made in discovery of dark energy is in error

#237

Post by Certhas » Tue, Jan 07, 2020, 8:30 PM UTC

shpeak wrote: post #236

Certhas wrote: post #230

Earlier quoted context omitted.

What's your point? Do you want me to explain the basics of GR and Cosmology, including the vast amounts of evidence we have for everything in HN comments? Do you think you are pointing out subtle errors in reasoning that none of the tens of thousands of physics students since the early twentieth century spotted? You don't have the decency to try to learn the basics but presume to lecture me/expect explanations? Take…

I'm trying to point out, that current evidence can be explained in different way: by kind of Tired Light Theory. TLT plays well with Pilot Wave Theory and it doesn't need epic events in the past just few galactic hours ago. I'm familiar with evidence used by expanding Universe theories and with problems in them. TLT predicts that value of "Speed of expanse" will be different when measured using different methods or f…

So yeah, your agenda is not to learn or discuss but to push falsified fringe theories, that were exhaustively discussed until the eighties, when better data ruled them out...

Re: Evidence that the key assumption made in discovery of dark energy is in error

#238

Post by blechinger » Tue, Jan 07, 2020, 8:41 PM UTC

dieppe wrote: post #90

bananabreakfast wrote: post #77

Earlier quoted context omitted.

Speaking from the pov as a former astrophysicist, that definition of redshift is not quite accurate. While we have observed the universe expanding since Edwin Hubble published his findings 90 years ago, it was in fact the Friedmann equations produced 10 years earlier that established how it would work within the context of general relativity. The Universe is expanding at every known point at the same time at the same…

> The Universe is expanding at > every known point at the same > time at the same rate This part I don't get (I am in no way versed in astrophysics). If I am in a box, and that the box grows 10%, but that at the same time I grow 10%, what has changed? I mean, if every point in the universe expands at the same rate at the same time, what does it change? Relatively speaking, everything is the same size before and after…

I am not a physicist.

My understanding is that the fundamental forces are strong enough to resist the local expansion of space.

So the box gets bigger but particles do not get ripped apart or themselves expand. Space is expanding. Not matter. The particles occupy the same space and the fundamental forces keep them together. Space just keeps moving and expanding around them.

This (coupled with the fact that, at least currently, the expansion is happening very slowly) is why the expansion isn't observable unless you're looking at a very large scale.

Re: Evidence that the key assumption made in discovery of dark energy is in error

#239

Post by shpeak » Tue, Jan 07, 2020, 8:53 PM UTC

Certhas wrote: post #237

shpeak wrote: post #236

Earlier quoted context omitted.

I'm trying to point out, that current evidence can be explained in different way: by kind of Tired Light Theory. TLT plays well with Pilot Wave Theory and it doesn't need epic events in the past just few galactic hours ago. I'm familiar with evidence used by expanding Universe theories and with problems in them. TLT predicts that value of "Speed of expanse" will be different when measured using different methods or f…

So yeah, your agenda is not to learn or discuss but to push falsified fringe theories, that were exhaustively discussed until the eighties, when better data ruled them out...

Yep. I was too young in eighties to participate in these discussions, so I can read about them only, and watch some lectures, which is not satisfying enough for my curiosity.

Can we discuss something easier?

Why you think that photon is immortal? Why we have rule of right hand in EM? Why we see star formations older than BB? WTF is "physical vacuum"? What is waved by gravitational waves? What is happen in linear Sagnac interferometer? How photon is formed (it requires FTL to form)? What happens in double slit experiment? And so on.

Re: Evidence that the key assumption made in discovery of dark energy is in error

#240

Post by cygx » Tue, Jan 07, 2020, 9:11 PM UTC

raattgift wrote: post #229

cygx wrote: post #148

Earlier quoted context omitted.

Kinematical interpretations fall apart really spectacularly I disagree: Relativistic frequency shifts (doppler shift, gravitational shift, cosmological shift,...) can all be understood kinematically: The invariant quantity you have to consider is photon 4-momentum, with energies at time of emission and absorption given by the projections onto the 4-velocities of the respective observers, so it's all about the relativ…

I'm pretty sure we were talking about the ensemble of redshifts across the sky, rather than one particular photon. (Even in the single-photon case there will be lots of uncertainty about the \lambda_{obsv} vs \lambda_{emit} relationship; think in particular about its interaction with ions along the way. We'd want at least an "ensemble" of photons along the lines of a classical beam equipped with a spectrum). I'm not…

In arbitrary spacetimes, redshift has to be computed along specific worldlines, ie you tend to treat photons as point particles. I guess in principle, you could solve the Einstein-Maxwell equations (or more likely, the Maxwell equations on a fixed background), but I wouldn't recommend it. There are special cases which are simpler to handle (eg if your spacetime is static and you use adapted coordinates, the time coordinate will be cyclic in the particle Lagrangian, the canonically conjugate energy will be conserved and redshift will be given by the quotient of the roots of the g_{00} components, which formally comes in via normalizing emitter and observer 4-velocities).

I'm not sure if you are objecting to approaches like the tetrad formalism in your final sentence

The final sentence was just a comment on the pedagogy of deriving cosmological redshift via infinitesimal Doppler shifts. There's nothing wrong with it (sections of the frame bundles and all that jazz if you want to get formal), but to me, this feels like putting things on their head: I consider the Doppler shift a special case of the generic relativistic shift, making it circular to then explain the generic case as a series of infinitesimal Doppler shifts integrated along a photon's worldline.