Dumb Dumb has a question (standard candles)

[Murphy625]

TL;DR

standard candle question

Hello, it's been a while since I've posted. Me (big dumb dumb) has a question.

Astronomers use Type 1A supernovas to gauge distance because these type of supernovas are generally all the same brightness due to the way they happen (hence the moniker Standard Candle).. White dwarf sucks hydrogen from a binary, reaches a critical mass, and boom. The explosions of these supernova are generally all the same brightness so we can calculate the distance to them based on how dim they appear to be from our position.

My question: Do we have (for lack of a better term) a standard speedometer? Would it be reasonable to assume that the ejecta material from the Type 1A supernova would all travel at the same speed as well? Could we measure the rate of expansion of the ionized gas from a Type 1A supernova?
I was thinking that the density of the interstellar medium around such an explosion would have some affect on the speed of the ejecta material, but I'm not sure how much effect it would have.

Now the part you're going to think is silly and makes me a dumb dumb. I've been reading about dark energy, dark matter, and the general history of why we think these things exist. Been reading on this subject for over ten years.. It makes no sense to me that the universe is only ~5% visible matter and the rest is magical invisible stuff we can't see.. Applying the principles of Occam's Razor, the simplest explanation is usually the correct one.

Could we have time dilation wrong? We're observing the universe from a gravity well (Earth), which is inside a gravity well (Our Sun), which itself is inside a gravity well (Milky Way Galaxy).. I think the technical term for what I'm describing is "constructive interference". Time slows down when you're in a gravity well.. which means to the observer in the gravity well, everything outside looks like it's moving faster than it really is. We measure the speed of these galaxies rotating, and we think they're too fast.. We measure the speed of other things in the cosmos, and they're all moving too fast... and we point to dark energy or dark matter as an explanation.. and all speed measurements are based on a time function, so if we have time dilation wrong and our clocks are slower than we think, then that would be a big problem.

When I started wondering about this, I figured that folks a lot smarter than I'll ever be would have already figured it out by looking at some standard speedometer here in our own galaxy and my thoughts were probably wrong. Imagine my surprise when I found out we haven't had any supernova in our galaxy for several hundred years.. long before any tech to measure anything. Which brought me to another point.. When I googled how often supernovas go off in an average galaxy, the number I got was at least 1+ per century.. and yet the last one in our galaxy was Kepler's in 1604. That's over 400 years. Once again, pointing to everything outside happening faster than at home. I understand that we might not see one going bang on the other side of the Milky Way, but there should have been five or six of them by now and it seems unreasonable to suggest they're all going off on the other side.

Comments? Anyone have any thoughts on this?

 

[Ibix]

The time dilation factor between here and infinity is, roughly speaking, $1+\frac{GM}{c^2r}$ where $r$ is the distance you are from the center of an object. For the Earth, that works out to an error on the tenth decimal place. The Sun, on the 8th decimal place. The effect is nowhere near big enough to be responsible for the observations.

More generally, the problem isn't just that the galaxies rotate too fast, it's also that the speed they rotate varies depending on the distance from the center, and the way it varies is wrong if there's only the stars there. So no single calibration factor can fix it.

Serious scientists have proposed solving the dark matter issue by suggesting that our theory of gravity might be wrong, but also by suggesting that it might be right but we've applied it wrong (by neglecting something that would be a measurable effect). As far as I'm aware they're very much minority opinions, but ideas of that nature are being investigated.

 

[Bandersnatch]

If our star produced such a deep gravity well that time dilation were significant, then you'd see all velocities speed up, at all galactic radii - instead, the inner velocities are just about right, while the outer are too high. If the argument were to use the differences in the depth of the gravity well of the galaxy, then the inner velocities - where the well is the deepest - would look too slow, and they don't.
Plus, there are other lines of evidence for a dark matter-like effect, including the Bullet Cluster, and probably most importantly the distribution of inhomogeneities in the CMB.

Similarly, with the dark energy, it's not that the effect produces faster velocities everywhere, or should I say 'everywhen' - the measured velocities are higher than expected during the recent epoch, and about as expected before that. This tracks with the universe decelerating when it was denser, and accelerating when a dark energy becomes dominant. What it doesn't track with, is a single factor bumping up all recessional velocities.

Furthermore, if gravitational time dilation were a factor, you'd necessarily observe it everywhere. From binaries and exoplanets not following Kepler's laws, to the universe being generally blueshifted, to stellar spectral lines showing strong dependence on how deep in the well they were emitted from.

Also, Kepler's supernova wasn't the most recent one in our galaxy. It was the most recent one able to be observed with the naked eye.

And as for the actual question that was posed, I'm not aware of any 'standard speedometers' nor am I sure it'd make sense to use them. You use standard candles as proxies for distance, since you can infer it from their constant brightness. Velocities you can just measure directly, using the Doppler effect on the spectral lines.
I'm thinking, maybe the spectral lines would fit your idea of a universal speedometer, since their lab-frame position tells you when the speed is zero.
But then again, you'd have to accept that gravity, while able to affect them in principle, is too weak to do so noticeably in almost all cases. Otherwise you'd be back on the same train of thought as in the OP, I'm guessing.

 

[Murphy625]

Thank you both for the excellent replies. Very clear and awesome answers.