Extraordinary claims require extraordinary evidence. This saying, popularized by the late Carl Sagan, represents a useful attitude to maintain when confronted with revolutionary claims. For example, if some group were to claim that a previously unknown property of space caused it to expand more quickly as space grew larger, that group should produce compelling evidence to support the claim.
Oh wait. The High-z Supernova Search Team and the Supernova Cosmology Project (SCP) both made such a claim over a decade ago. The fact that two independent teams of astronomers reached the same conclusion added weight to the evidence that the groups published in their respective papers. However, the far-reaching implications of this research elicited a measured response from the scientific community. While a majority of astronomers and cosmologists now accept the idea that space exhibits this "anti-gravity" property, they still seek to verify and understand it.
The original results from the High-z team and the SCP relied on measurements of distant type Ia supernovae. Supernovae at higher redshifts were dimmer than expected, implying that the expansion of the universe started accelerating roughly 5 billion years ago. However, if the universe's expansion were accelerating due to a mysterious "dark energy," other observations should provide confirmation.
Scientists' confidence in the original research was greatly enhanced by one significant discovery that utilized a large survey of galaxies taken by the Sloan Digital Sky Survey (SDSS). As described in an earlier TNRTB, this "dark energy" would affect the clustering of galaxies across the universe. In 2005, a team of astronomers detected the signature of dark energy, known as the baryon acoustic peak. Recently, the SDSS released a newer catalog with more than twice the number of galaxies. A careful analysis (with more accurate handling of errors) of this catalog confirmed the presence of the baryon acoustic peak.
Along with the supernovae results and the galaxy clustering results, data from the cosmic microwave background also confirms the existence of dark energy. As noted by numerous scientists, the dark energy represents one of the most extreme fine-tuning problems known. Consequently, some cosmologists seek explanations that don’t require dark energy. One model published in the Proceedings of the National Academy of Science argues that a "big wave" model explains the supernovae data without any need for dark energy. But it remains to be seen whether this new model also accounts for the SDSS and CMB data.
For now, the bulk of the evidence indicates that we live in a universe composed predominantly of the extraordinarily fine-tuned dark energy. The level and degree of fine-tuning defy naturalistic explanations, but validate a model that posits a supernatural Creator.