According to our current understanding of physics, there is no universally agreed-upon notion of "now" that applies simultaneously across the entire universe. This concept is rooted in the theory of relativity, specifically in the relativity of simultaneity.
In special relativity, it is established that the concept of simultaneity depends on the observer's reference frame. Different observers in relative motion will have different perceptions of what events are happening "now" at a given moment. This effect becomes more pronounced as the relative velocities involved approach the speed of light.
In the context of the entire universe, which is vast and contains objects at various distances, it becomes even more complicated. Light travels at a finite speed, so when we observe distant objects, we are seeing them as they were in the past because the light takes time to reach us. The farther away an object is, the greater the time delay in our observations.
For example, if we were to observe an event happening on the other side of the universe, it would take a significant amount of time for the light from that event to reach us. By the time we receive the light, the event itself may have already occurred in the distant past. Therefore, we cannot take a "freeze frame" of the entire universe and know with certainty what is happening simultaneously across vast distances.
It's important to note that the concept of "now" is a subjective experience for local observers within their reference frames. However, from a universal perspective or a cosmic scale, there is no absolute "now" that encompasses the entire universe at once.