Question

If we say that a galaxy has a look-back time of 2 billion years, we mean?

Answer 1

The look-back time of 2 billion years for a galaxy indicates we are seeing the galaxy as it existed 2 billion years in the past due to the time it takes for light to travel to us. This concept is a crucial tool for astronomers to study the evolution of galaxies over the universe's history.

Step-by-step explanation:

When we say that a galaxy has a look-back time of 2 billion years, we mean that we are observing the galaxy as it was 2 billion years ago because that's how long the light has taken to travel from the galaxy to us. This concept allows astronomers to see galaxies at various stages in their evolution by observing them at different distances, effectively creating a timeline that depicts the history of the universe and the life story of these galaxies. As we observe galaxies further away, we're looking further back in time, giving us insights into the early universe and the formation and evolution of galaxies.

For galaxies close to us, like Andromeda, the changes over timescales of a few million years may not be significant, but by studying extremely distant galaxies, we can witness the conditions of the universe as it was over 10 billion years ago. By comparing galaxies at various look-back times, astronomers can piece together a picture of galactic evolution, from 'baby' galaxies in the early universe to mature 'adult' galaxies in the current 13.8-billion-year-old universe.

To further understand galactic evolution, astronomers also study the oldest stars in our own Galaxy, which serve as a kind of 'fossil record' for the history of the universe. These observations contribute to building our knowledge of the cosmos, despite the significant uncertainties that are inherent in cosmology.

Answer 2

A look-back time of 2 billion years means we see the galaxy as it was 2 billion years ago, offering insight into the galaxy's evolution and the universe's history, by observing it at different stages of its lifecycle.

Step-by-step explanation:

If we say that a galaxy has a look-back time of 2 billion years, we mean that we are observing the light from that galaxy as it was 2 billion years ago. This concept is foundational in understanding the evolution of galaxies and the cosmic history. It serves as a cosmic time machine, providing us with a glimpse into the past of the universe, similar to peering through an immensely vast historical archive.

When we observe distant galaxies, we are effectively looking back in time because light travels at a finite speed of approximately 300,000 kilometers per second. Thus, the further away a galaxy is, the longer its light has been traveling before reaching us. For example, the Andromeda galaxy's light has been traveling for about 2 million years to get to us, revealing how Andromeda looked 2 million years ago.

The ability to look back over billions of years allows astronomers to study the formation and life cycle of galaxies by comparing different galaxies at various stages of their evolution. This comparative analysis can help piece together a coherent narrative of galactic history. Overall, the study of light reaching us from different distances provides a critical window into understanding the structure and development of the universe, from its earliest stages to its current state.