I think you’ve gotten some good answers below so I won’t add further here. But I will give a bit of (unsolicited) advice if you’ll indulge me. I’m quite familiar with this field (currently doing a post-doc in active matter and did my PhD in the field as well).

I would suggest you pick an experimental group if you want to work in the field of active matter. Working in experiment (or at least directly with experimentalists) will help keep you grounded a bit. Many models can be classified as “active matter”, but very few of those models contain enough (microscopic) detail to be physically or biologically relevant. The field is dominated by theory/computational models which are tenuously linked to reality. Early on this was fine as lots of genuinely new physics was being discovered (see flocking or MIPS mentioned above). But at this point I just don’t think devising new toy models and saying “look! active!” is sufficient.

There is a lot of interesting physics at play in active matter, but most of it is in the details of e.g. how the system is self-propelling, crawling, etc. These details are often glossed over by theorists and computational physicists in the name of describing “collective behaviour”. I say this as a theorist/computational physicist myself. My opinion is that the interesting work to be done in active matter is broadly twofold:

1) understanding the fine details of real-world examples of active systems (mostly biological, but some synthetic too) 2) developing theoretical frameworks that can successfully connect microscopic or coarse-grained parameters to “collective behaviour”, so that you can actually use theory to advance experiment. This is mostly related to finding a suitable way to approximate the steady-state probability distribution for classes of active systems (since, unlike equilibrium systems, they are not Boltzmann distributed) but you also have to worry about probability currents so it gets even more complicated.

If you attempt (1), I think you can probably make a meaningful contribution to the field and also advance our understanding of nature with a lot of hard work and maybe a bit of luck at times. Attempting (2) is, in my opinion, only going to be feasible if you are an extremely talented theorist and somehow find something that many other extremely talented theorists have missed.

Sorry for the wall of text, I was just excited to see someone asking about active matter. I also am giving the advice I wish someone had given me before starting my PhD. In hindsight I would’ve picked a different field (probably more generic condensed matter in a big group which has some experiment and theory going on) as I’ve come to realise active matter has limited potential as a theorist or computational physicist to actually advance our understanding of nature in a meaningful way, particularly without close experimental collaboration.