Sixth milestone: 606 light years from Earth
At the very edge of the observable Universe, more than 45 billion light years* from Earth, we glimpse the
Cosmic Microwave Background Radiation (CMBR): the faint echo of the
Big Bang itself. The image shown above is an all-sky
CMBR map produce by NASA's
WMAP satellite, launched in 2001.
This light comes to us from about 380,000 years after the Big Bang, when the universe was much smaller and denser than it is today. The CMBR carries the imprint of tiny cosmic "ripples" that were present in the very early Universe, and which gravity has amplified over billions of years to make the galaxies we see today.
By studying the CMBR we can trace the history of how the universe has expanded, and perhaps better understand how it might evolve in the future.
Image credits: NASA WMAP Science Team
* Note that, because of the finite speed of light, when we look at distant parts of the Universe we are seeing them as they were a long time in the past. Consequently, when we say that the CMBR is more than 45 billion light years from the Earth we are expressing the distance to the CMBR now, after accounting for the (considerable) expansion of the Universe that has taken place since the CMBR light we see today was emitted.