Are there no cosmic sources of randomness, for example variations in pulsar frequency? That could solve the "No older than this" part, by imprinting a sufficie

Are there no cosmic sources of randomness, for example variations in pulsar frequency? That could solve the "No older than this" part, by imprinting a sufficiently long (unique) sequence from a particular time on your asset. Although I suppose it would be possible for whoever keeps the recorded "event-log" to just alter it. For the "No younger than this" part, it seems you need to do the reverse, imprint the universe with your signature, which can then be verified by anyone who looks. I guess blockchain can work as "universe" to some extent, but a more natural/absolute solution would be neat.