…there are two different ways to measure this cosmic expansion rate, and they don’t agree. One method looks deep into the past by analyzing cosmic microwave background radiation, the faint afterglow of the Big Bang. The other studies Cepheid variable stars in nearby galaxies, whose brightness allows astronomers to map more recent expansion.
You’d expect both methods to give the same answer. Instead, they disagree—by a lot. And this mismatch is what scientists call the Hubble tension…Webb’s data agrees with Hubble’s and completely rules out measurement error as the cause of the discrepancy. It’s now harder than ever to explain away the tension as a statistical fluke. This inconsistency suggests something big might be missing from our understanding of the universe - something beyond current theories involving dark matter, dark energy, or even gravity itself. When the same universe appears to expand at different rates depending on how and where you look, it raises the possibility that our entire cosmological model may need rethinking.
i agree with you that here, the difference between 68 and 73 seems very small.
For me, it’s even amazing that they get, for the CBM, any number even close to the same order of magnitude, given that it seems like a linear division of speed of light divided by light travel distance at the age of the universe, is the value for Hubble parameter (H)*_ at CBM.
That seems in contradiction to the fact that, when adding relativistic velocities (and incrementally up to the speed of light !), linear addition is out of question and general relativity has to be used.
This is just one of the apparent difficulties and obviously there are much more and harder challenges than this one.
_*(… and is the age of the universe defined or measured by other means than simply :
Δt = 1/H … ? That can’t be : since we have 2 parameters to evaluate, so, we need 2 independent experimental measurement variables. )
The majority of physics is done with very high precision. This is especially true for fundamental values that apply to everything. For example know the mass of an electron with an error of 0.3 parts per billion. I think this discrepancy is evidence of a significant hole in cosmology theory.
Yes i agree that, doing physics in a very well controlled laboratory, physicist can measure things accurately.
Unfortunately we don’t have a laboratory big enough to reproduce a big bang and study it in a controlled fashion. So, in cosmology, measurement are difficult and not so precise 😋.
Lambda-CDM is fully aware of general relativity. Some people may try to explain it with nonrelativistic pictures to help you build intuition, but the actual theory and calculation is fully relativistic so you don’t have to worry about that.
I don’t follow. What two parameters?
Yes i agree with you that, of course, physicists working on this have to be well aware of general relativity. Still, there is this linear relationship that bothers me for the Hubble parameter.
i should have put more effort in understanding before writing my comment … and this confusion about “two parameters” is nothing of importance for what i try to say in that comment. Sorry if you don’t see anything interesting in what i said.
Any linear relationship in this calculation would be an approximation. They’re useful for intuition and quickly explaining things, but for actual business either the full nonlinear relationship is used, or if the linear approximation is used the approximation error must be bounded by an acceptably small parameter.
From previous articles, I do believe it is consistent enough across different approaches and precise enough that there really seems to be more than one answer. How can that be?
We really don’t have a solid reason for the increase in expansion rate of the universe. Dark energy seems most straightforward and consistent with everything else but it’s not proven until we can identify and measure that energy. This lends weight to the idea that it’s not that simple
Suppose we come to establish that the expansion rate accelerated from 68 to 73 km per second / Mpc (in the lapse of, say, 80% the ~14 B. years age of U.) if this is so, so be it. Why oppose it ?
Or, if for the same period, we have two different rates … this is not acceleration. This is two methods yielding different results for what is supposed to be one sigle thing. So, one of the 2 methods doesn’t measure exactly the same thing as the other … whatever.
Obviously, observation and measurement have to be the basis for any hypothesis and for any explanation proposals. So, we should not say : “since we have no explanation, there should not be acceleration of the expansion”. - - But rather we should say : “since there is acceleration of the expansion, we should build some theoretical models around this reality”.
Anyway, you probably already know all of this.
Right. Theoretical models helped us get here, where we identify specific criteria to test. However two different answers don’t fit our current models of the universe. Something has to change: either the answers do not mean what we think or the universe evolves differently than we think
It’s fascinating how complex and wonderful it it is that every time we think we have something figured out, nature gets more complex
It’s a real life example of HGttG:
i love Douglas Adam’s trilogy in five books as well 😁