# Has the rate of time passage been measured?

We know certain astronomical events in our neighborhood occur within a specific time interval,for example a solar flare, a nova, etc. Does the time interval of these 'standard metronomes' change if they are further away (millions of light years?)

• An important fact about things million of years away is that they are moving relative to us, so experience relativeistic time dilation. There is a standard clock to use, the rate of decay of nickle56 as this causes the brightness of supernovae to change at a regular rate. As far as I recall, the predictions of relativity are supported by these observations, that is, there is time dilation for objects moving at high velocity, but not for merely being in a different place, I'll try to find where I read this. Commented Apr 12, 2023 at 15:10

1. Type Ia supernovae have decay light curve widths that are reasonably standardised (there is some dispersion). However, when the widths of distant supernova light curves are observed, they appear to be dilated by a factor $$(1+z)$$, where $$z$$ is the redshift (e.g., Blondin et al. 2008).
2. Another piece of evidence comes from Gamma Ray bursts under the assumption that they should have (statistically) a similar mean duration at any point in the universe. Zhang et al. (2013) find that the duration of "long" GRBs at known redshifts (and measured in the same rest-frame energy) varies as $$(1+z)^{0.94±0.26}$$, consistent with cosmological time dilation. See also Littlejohns & Butler 2014.
• @aquagremlin if the amount of time dilation was a factor of 1 (i.e. independent of redshift), then it would be evidence for the so-called "tired light" explanation of the redshift. It isn't. Interpretation for dilation of $(1+z)^\alpha$ where $\alpha \neq 1$ could be that the nature of these events has changed over cosmic time. The speed of light is defined to be constant, it is meaningless to discuss a change in the speed of light; only dimesnsionless constants can show evidence for variation. Commented Apr 13, 2023 at 8:17