Final answer:
Downticks on timelines are markers of significant events or periods in chronological relation to other events, helping to graphically illustrate the progression of time from the past to the present.
Step-by-step explanation:
The downticks mentioned on timelines typically represent specific events, dates, or periods of time that are significant within the overall scope of the timeline. A timeline serves as a visual representation of chronological events, making it easier to understand the sequence and relation of one event to another. In the context of the provided information, where there's a white arrow pointing from the 'Big Bang' to the 'Present', downticks could mark important milestones or epochs in the universe's history. The purpose of these downticks on the timeline is to provide a clear and precise outline of when certain key events occurred, allowing the viewer to determine graphically the progression of time from the past to the present.