Direct evidence for an accelerating Universe came from observations of type Ia supernovae by the High-Z Supernova Search Team (Riess et al. 1998) and by the Supernova Cosmology Project team (Perlmutter et al. 1999).
Their research showed that remote supernovae are 10% to 25% dimmer and therefore further away than expected as compared to the nearby (local) supernovae.
Standard luminosities of type Ia supernovae helped the researchers to determine their distances, while the observed redshifts provided an estimate of their recession velocities – the relation between redshift and distance helped the researchers to determine the expansion rate (km/s/Mpc) of the Universe.
Remote measurement yields an expansion rate of 46 km/s/Mpc (Blanchard et al. 2003) which is much lower than the local measurement of 72 km/s/Mpc by the Hubble Key Project (Freedman et al.).
The expansion rate (km/s/Mpc) for remote supernovae is lower than the expansion rate for local supernovae, therefore, we say that the Universe is accelerating now and had a slower expansion in the past.
According to Durrer (2011), “our single indication for the existence of dark energy comes from distance measurements and their relation to redshift”.
Working on research made me analyse the data. The peer-reviewed paper presents a novel interpretation of the redshift-distance relationship of observed supernovae as reported by the scientific reviewer of one of the most prestigious astronomy journals.
I believe accelerating Universe is a surprising discovery due to an undiscovered aspect.
Answer to PM 2Ring’s comment:
The expansion rate for local structures ranges between 68 km/s/Mpc and 74 km/s/Mpc, whereas for remote structures, the expansion rate ranges between 40 km/s/Mpc and 60 km/s/Mpc.
What we don’t know is the current (present) expansion rate for the remote structures, and, it is simply assumed that the expansion rate derived from the local structures is the present expansion rate for the entire Universe.
According to the Copernican principle, we are not any special or privileged observers, therefore, enforcing or simply assuming that the expansion rate for the entire Universe is the same as the expansion rate for the local Universe also appears to conflict with the Copernican principle since the expansion rate for the local Universe is being prioritized over the entire Universe.
It is not correct to assume that the expansion rate for the entire Universe is the same by prioritizing the local expansion rate over the entire Universe without actually knowing the current (present) expansion rate for the remote structures.
Direct evidence for an accelerating Universe came from observations of type Ia supernovae that showed that remote supernovae are further away than expected as they appeared 10% to 25% dimmer than the local supernovae.
Possibilities included pervasive screen of grey dust between the local and the remote Universe, and the evolution of type Ia supernovae. These possibilities have been addressed and are no longer a concerning factor.
If remote structures began expanding into the Universe before the expansion got initiated for the local structures, then in this case also remote structures would end up being further away than expected. This is exactly what we observe – remote supernovae are indeed further away than expected as compared to the local supernovae.
Now, to prove this that remote structures began expanding into the Universe before the expansion got initiated for the local structures we require a confirmation that would test this possibility.
Direct confirmation for this possibility comes again from analysing those direct observations of type Ia supernovae that made the research team conclude that the Universe is accelerating.
Remote structures are not only further away than expected, but they also yield a slower rate of expansion even with high recession velocities (recession velocities ranging from 30% to 60% of speed of light) as compared to the higher rate of expansion for the local structures even with low recession velocities (recession velocities ranging from 1% to 10% of speed of light).
There can’t be any other reason for such a trend where an object with high recession velocity is not only further away than expected, but is also yielding a slower rate of expansion as compared to the expansion rate obtained for an object with low recession velocity. This is only possible if remote structures began expanding into the Universe before the expansion got initiated for the local structures.
The expansion rate for remote structures ranges between 40 km/s/Mpc and 60 km/s/Mpc – it is not the same for all remote structures, but depends upon their distance and recession velocity, or more precisely when they began expanding into the Universe.
It is not possible that all structures would have expanded at the same time into the Universe, objects with high recession velocity that began expanding before are further away than expected and yield a slower rate of expansion, whereas objects that began expanding comparatively later yield a faster rate of expansion.
The manuscript has been revised; the results have been confirmed further by plotting,
- Expansion rate vs. time relationship,
- Expansion factor vs. time relationship, and
- Scale factor vs. time relationship