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

#191

Post by cygx » Tue, Jan 07, 2020, 1:00 PM UTC

igravious wrote: post #88

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…

One thing I've never understood is this. How do we tell the difference between the expansion of space and the dilation of time? Given that space and time are crudely specking intertwined (spacetime) how do we know that the effect we see -- redshifting -- is due to the spatial expansion at every known "point" and not the temporal expansion (contraction?) at every known moment? Could not the spatial size of the univers…

Relativistic frequency shifts can be understood as kinematic effects, namely orientation of photon 4-momentum relative to emitter and absorber 4-velocities.

Now, spacetime geometry might be such that there exists space/time decompositions that allow us to attribute the shift to specific physical effects (relative velocity of emitter and absorber in case of special-relativistic Doppler shift, time dilation in case of gravitational shift in Schwarzschild spacetime, spatial expansion in case of cosmological shift in FLRW spacetime), but from the perspective of spacetime geometry, these are just specific instances of a general mechanism that is agnostic to such attributions.

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

#192

Post by shpeak » Tue, Jan 07, 2020, 1:07 PM UTC

brylie wrote: post #54

brylie wrote: post #53

Earlier quoted context omitted.

I'm not an astronomer, but "tired light" makes more intuitive sense to me to explain the redshift. In effect, we already know that matter can affect light through gravitational interaction. Since light is travelling so fast and far it seems intuitive thst interstellar matter, as fleeting as it is, could act to slow light down over these great spacetime distances.

According to the Wikipedia article, "tired light" has been discredited due to lack of observational evidence and assumptiins about light scattering: https://en.m.wikipedia.org/wiki/Tired_light

Light scattering assumption is invalid, this is true, so photon losses energy because of an other mechanism.

However, newer telescopes discovers more stars and galaxies than predicted by theories of expanding universe, thus favors Tired Light Theory. Also, discovery of gravitational waves confirmed that (speed of light in pure vacuum) c≠const, so physical vacuum is not an empty space, thus favoring TLT again.

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

#193

Post by shpeak » Tue, Jan 07, 2020, 1:11 PM UTC

CoryG89 wrote: post #62

brylie wrote: post #53

Earlier quoted context omitted.

I'm not an astronomer, but "tired light" makes more intuitive sense to me to explain the redshift. In effect, we already know that matter can affect light through gravitational interaction. Since light is travelling so fast and far it seems intuitive thst interstellar matter, as fleeting as it is, could act to slow light down over these great spacetime distances.

This doesn't make much sense to me. The speed of light is a constant, right? Gravitational interactions may change the directional component, but the speed would remain constant, no?

No. Gravitational waves are causing fluctuations of speed of light in pure vacuum. (See LIGO/VIRGO experiments). So c is constant in steady physical vacuum only, i.e. it's speed of EM wave in medium. If you will change properties of medium, then c will be changed too.

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

#194

Post by urbanjunkie » Tue, Jan 07, 2020, 1:21 PM UTC

shpeak wrote: post #189

DavidSJ wrote: post #185

Earlier quoted context omitted.

This isn’t Certhas’s personal theory. It’s part of standard cosmology, called the metric expansion of space . That term should get you pointed in the right direction if you want to learn more about the evidence for it.

It's too complex to explain, I will not understand it. Right?

Literally Google "metric expansion of space"

Dickhead

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

#195

Post by ses1984 » Tue, Jan 07, 2020, 1:22 PM UTC

galaxyLogic wrote: post #164

roywiggins wrote: post #103

Earlier quoted context omitted.

So, imagine the box is filled with hydrogen gas. The space inside the box gets bigger. But protons don't, they're little packets of quarks that hold each other very powerfully. You'd have to expand space very, very, very fast to rip those quarks apart from each other, and once you did, you would have a bunch of bare quarks, not hydrogen. The quarks aren't getting bigger, since they appear to be indivisible and essent…

Well so in other words "space" is NOT expanding. It's particles of matter that are moving away from each other. Right? So fine, but then why would we say that space is expanding, that would seem to imply we know there is some edge of space which is moving away from us, but what evidence do we have that space has such an edge? Saying that "space is expanding" seems to hold the assumption that space has a (limited) wid…

If things were moving away from each other, there would have to be something that set them in motion away from each other. What made all far apart galaxies appear to move away from each other?

Expanding space doesn't necessarily imply edge of space. What if your universe isn't flat, what if it's a torus? It could hypothetically have a finite volume but no edge.

There's also no reason an infinite universe couldn't have local expansion everywhere.

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

#196

Post by lmilcin » Tue, Jan 07, 2020, 1:39 PM UTC

galaxyLogic wrote: post #164

roywiggins wrote: post #103

Earlier quoted context omitted.

So, imagine the box is filled with hydrogen gas. The space inside the box gets bigger. But protons don't, they're little packets of quarks that hold each other very powerfully. You'd have to expand space very, very, very fast to rip those quarks apart from each other, and once you did, you would have a bunch of bare quarks, not hydrogen. The quarks aren't getting bigger, since they appear to be indivisible and essent…

Well so in other words "space" is NOT expanding. It's particles of matter that are moving away from each other. Right? So fine, but then why would we say that space is expanding, that would seem to imply we know there is some edge of space which is moving away from us, but what evidence do we have that space has such an edge? Saying that "space is expanding" seems to hold the assumption that space has a (limited) wid…

Space is expanding but gravitational attraction and other forces are many orders of magnitude stronger on short distance. While the space within the box expands the forces that attract molecules of the box are such that it will not change its shape until the expansion gets much, much stronger.

Think of the changing pressure. If the box is tight and rigid and the pressure outside the box increases, the box will roughly stay the box until the pressure overcomes the the forces between the molecules that build the walls.

Same with space. If you could measure, the increase in the volume of space would put a constant pressure on every molecule to try to rip it from other molecules. The fact that the rate of change of space and the distances between molecules are so small means that it is not evident that it will ever be possible to devise an experiment to show this on very short distances.

We can only see this on very long timescales and between objects that are very far from each other because all other forces on those scales have diminished so much that only then the increase of volume of space can dominate.

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

#197

Post by fennecfoxen » Tue, Jan 07, 2020, 1:39 PM UTC

inakarmacoma wrote: post #126

fennecfoxen wrote: post #102

Earlier quoted context omitted.

Consider yourself. Consider a galaxy N million lightyears away that is moving away from you at high speed... Or is it really? What if it's staying perfectly still, and YOU'RE the one moving? ... How could you even tell? The principle of Relativity is that you cannot tell any difference, for the laws of physics are the same whether you are moving or the galaxy is moving. The theory of Special Relativity describes whic…

... at which point you also get to enjoy your arrival at linguistic metaphysics. If you can identify all material objects that all observers can feasibly see, then you subtract the material objects that one person sees, from the material objects the other person sees in their reference frame, and you end up with... something... that is both a material object, and not a material object at the exact same point in time.…

> If you can identify all material objects that all observers can feasibly see, then you subtract the material objects that one person sees, from the material objects the other person sees in their reference frame...

Generally speaking an event is "observable" if some signal (usually light) could reach you from that event. It is generally uninteresting that different observers can see different sets of events at different points in their world-line. If you are lightyears apart, the signals will take time to reach you. This is what "spacelike separation" is all about.

But if you just sit around in one spot, signals can reach you from billions of light-years away. A certain event in your history might have been separated by space, but a point in your future may be separated from it in a timelike manner. You can't get away from events by simply going fast, either: the velocity addition makes signals impossible to outrun. What you need is to employ acceleration.

Consider a modified Zeno's paradox: you are 1 light year away from a signal, and begin accelerating. By the time it has covered 0.5ly, you are now 0.75 ly away. Unlike the original paradox, the math on this one works out: it is the acceleration equivalent of a black hole. (Most gravity and acceleration effects are interchangeable like that).

We could draw this out with a Minkowski diagrams, and find some line beyond which events become truly unobservable. We could call this line the "event horizon" (which, after all, is what the term is about in other contexts, too.)

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

#198

Post by ash » Tue, Jan 07, 2020, 1:56 PM UTC

ash wrote: post #152

j-dr wrote: post #118

Even if this paper is correct, there is still overwhelming evidence for dark energy provided by other observations, in particular those of the Cosmic Microwave Background (CMB) and Baryon Acoustic Oscillations (BAO). This article provides some details on why Dark Energy is in no danger of being called in question: https://www.quantamagazine.org/no-dark-energy-no-chance-cosm...

As far as I can tell, Quanta Magazine article does mention the original post findings. Additionally, the Quanta article and the findings preprint were published just a week apart. Not enough time to formulate a good response: * Quanta Magazine article: 17 Dec 2019 * Arxiv preprint: 10 Dec 2019 [1] CMB and BAO are mentioned in the original post too: > Other cosmological probes, such as the cosmic microwave background…

Correction: "Quanta Magazine article does NOT mention the original post findings"

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

#199

Post by roywiggins » Tue, Jan 07, 2020, 2:23 PM UTC

galaxyLogic wrote: post #164

roywiggins wrote: post #103

Earlier quoted context omitted.

So, imagine the box is filled with hydrogen gas. The space inside the box gets bigger. But protons don't, they're little packets of quarks that hold each other very powerfully. You'd have to expand space very, very, very fast to rip those quarks apart from each other, and once you did, you would have a bunch of bare quarks, not hydrogen. The quarks aren't getting bigger, since they appear to be indivisible and essent…

Well so in other words "space" is NOT expanding. It's particles of matter that are moving away from each other. Right? So fine, but then why would we say that space is expanding, that would seem to imply we know there is some edge of space which is moving away from us, but what evidence do we have that space has such an edge? Saying that "space is expanding" seems to hold the assumption that space has a (limited) wid…

Every cosmologist since Hubble who has held that space is expanding, no, they don't assume the universe is finite. The opposite, in fact.

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

#200

Post by mannykannot » Tue, Jan 07, 2020, 2:24 PM UTC

bananabreakfast wrote: post #77

flashman wrote: post #34

To know if the universe is expanding, we need to know the following: how far away things are, and whether they're moving away from us. In an accelerating universe, there's evidence that things were moving away from us slower in the past. The evidence that things are moving away from us is that as the universe expands, the wavelength of any light travelling through it also expands, shifting it towards the red end of t…

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