Ever since Adwin Hubble discovered, that the universe is expanding, there has been a great debates about speed and expression also called the Hubble constant. 


In 2000, scientist thought they had reached a consensus: with our Hubble constant at about 72, but since then, two methods of calculating the expansion rate have gotten further and further apart. Scientist thinks it's too big of a difference to just be a statistical fluke. 


The calibration of the Hubble constant, we need velocity and distance. It requires a measuring the redshift of a galaxy and it's recedes. But distance are much harder. We need standard candle, objects that shine with an intrinsic brightness. That's way, its observed brightness could be converted to a distance. 

The first method to calculate the Hubble constant, orignated with Hubble. Please started by measuring distance to nearby objects and working his way out, hence the name, "cosmic distance ladder" The first ring is with in our Milky Way. Here we measure the distance to pulsating stars that have an intrinsic brightness. Then we look to nearby galaxies that contain both cepheid and another type of standard candle,


Supernova, these exploding with white dwarf stars also have an intrinsic brightness. After calibrating this supernova, Astronomers can then move out to third rung, to even more distant galaxies, that only contain supernova. Linking all these measurement, a recently study has pinned the Hubble constant at 73. Instead of starting with in our galaxy and looking out the second method looks back in time to almost the beginning of the universe. 


Where we find the cosmic microwave background: the radiation left over the big bang. The CMB's subtle variations in temperature describe changes in the density of the universe right after the big bang. Instead of the standard candles, physicists use the size of tiny fluctuations as standard yardstick. With these yardsticks, they calculate what the Hubble constant would be today. 


Recent estimates or at about 68, significantly lower than the cosmic distance ladders approach. 

Bahut campus hope to resolve this debate in the coming years with the help of new telescopes and instruments. But if the discrepancy in the Hubble constant  remains, it could mean that our understanding of the expansion of the universe is missing an ingredient that something happen between the Big bang and present. 

Maybe there's a neutrino out there we have not discovered. Maybe mysterious force dark energy, which makes up a significant portion of our universe, can change its strength. Or may be subtle flaws in one or both approaches to the Hubble constant and are giving us wrong answer.