How do we know that fundamental constants like the speed of light ©, Planks constant (h) , Gravitational constant (G), Electromagnetic constant (epsilon), etc. do not change with space/time ?
if they do change, how would it effect our current understanding of the Universe / Dark Matter / Black holes ?
We don’t know, but if we don’t make that assumption then all models go flying out the window with invisible unicorns.
I am fairly sure that there have been physical/cosmological theories seriously proposed in which certain of these constants do change over time. Relevant link
If the speed of light had been very much faster in the past, we would see distant events playing out in extreme slow motion.
Just like if you record something on magnetic tape at high speed, then play it back at a slower speed. Light in transit is the recording medium by which we observe distant objects.
Fundamental constants are like a house’s foundation – if they become unstable, or redefine themselves randomly, the Universe would probably cease to exist. Indeed, some constants like the Elementary Positive Charge (approx. 1.602176565(35)×10[sup]−19[/sup]) are required for baryonic matter to exist at all.
This isn’t true if you go back to the earliest stages of the universe.
Do we have evidence showing that distant events are not any faster or slower ? How would we even know ?
The question is a good one, but asked in the wrong way. It’s meaningless to speak of c or hbar changing, since those are the standards by which we define other constants: c has always been one lightsecond per second, for instance. To some extent, this is true of any constant which has units attached to it.
If you want to consider the possibility of constants changing, you need to look at the dimensionless constants, those with no units, such as the Fine Structure Constant (approximately 1/137). Most of those, it’s considered very unlikely that they’ve changed with time, but it’s conceivable that they might have, and in fact there have been some observations which can be interpreted as (weak) evidence that the Fine Structure Constant has changed.
Isn’t this something like asking if the sum of the angles of a triangle was a half a circle at the time of the big bang? Or if 7 was a prime number?
Not quite… It’s a little more like asking what is the sum of the angles of a triangle in bent space. How bent is out space? (Space-time.)
We’re pretty darn sure that fundamental constants have been unchanging for a long, long time. The most distant galaxies still show the emission lines of chemical elements without change from what we know right here at home in the chem lab.
But in the earliest femtoseconds of the big bang? Things might have been a trice dodgy them. The prevailing theory is that the fundamental forces – electromagnetic, strong nuclear, and weak nuclear – were “unified” in those earliest times, and so they wouldn’t have had numeric values at all. You couldn’t ask what the charge on the electron was…when there was no distinct electromagnetic force by which “charge” is measured or even defined.
Once the universe got really old – like, five or six seconds – things seem to have stabilized.
Well, we can directly observe the universe back to about 350,000 years after the big bang. When astronomers look at the cosmic microwave background they’re looking at the cloud of hot plasma that filled the universe right after it came into existence. If the fundamental constants have changed since then it should show up as anomalies in the CMB.
And there are some astronomers who claim to have observed such anomalies. They’re very small, though (implying a very small change), and even the authors of such papers don’t think that’s the most likely explanation.
I somehow read the title as "How do we know that consonants like c or h were the same at Big bang ? " - and am thus disappointed with how rational this thread has proved to be.
We can measure the speed of light that is travelling perpendicular to the direction of our view, at a distance - for example, by watching the light from a supernova illuminate other objects local to it.
I do not understand why it would be meaningless to speak of c changing +~10% to 399,000m/s or to speak of h changing +~10% to 7.288 x 10^-34js.
If the fine structure constant changed that would mean that one or all of its constituents did something. What would that something be if not “change”?
Make that 339,000,000m/s in my last post. I think. Even small numbers can confuse me.
You are misunderstanding. Its the other way around.
If the properties that we assume to be constant actually change, the definition of the day of the standards (eg perhaps of the meter) would become WRONG.
And a dimensionless constant just means that you have a ratio occurring between two values of the same dimension unit. there is no magic in that.
What does it mean to say that c is 3*10^8 meters per second? What is a meter? What is a second? Maybe the number of times a cesium atom can oscillate while a photon travels between two scratch on a platinum rod can change. But does that mean that c changed, or does it mean that the size of the platinum rod changed, or that the oscillation frequency of cesium changed?
Or to put it another way, in the most commonly used units, the value of c is 1. Can 1 change?
Google seems to relate that this interpretation originated with one V.A. Dzuba in 2002. If Googling tells us elswhere what mainstream consensus now is I have missed it. On the other hand there are plenty of more recent Google hits referring to “Change” in the speed of light. I interpret this to mean the issue is in doubt. Not so?
What Chronos is saying is that, if the speed of light is increasing, it could be because a meter stick is getting shorter or because a clock is slowing down. You couldn’t distinguish. Thus, to a physicist, the only meaningful statement is to specify which dimensionless ratios are changing, like the fine structure constant, or the ratio of the mass of the electron to the mass of the proton.
Dirac proposed such a hypothesis in the 1930’s, his so called large numbers hypothesis, which explained the weakness of the Gravitational force as being due to the great age of the universe. Observational evidence eventually ruled out his hypothesis, at least in its original form.